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Marsh & Mclennan Software Engineer Interview Guide

Marsh & Mclennan Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

Marsh & Mclennan is a premier global professional services firm specializing in risk, strategy, and people. As a Software Engineer at Marsh & Mclennan, you will play a critical role in designing, building, and maintaining high-performance software systems that power global risk management, insurance brokerage, and consulting platforms. The solutions you develop directly impact how multinational corporations analyze risk, allocate capital, and protect their workforces. In this role, you will work on enterprise-scale applications that require high availability, robust security, and seamless data processing capabilities. Engineering teams at Marsh & Mclennan are tasked with modernizing legacy systems, migrating core services to the cloud, and developing sophisticated APIs that integrate with diverse backend systems.

This guide is scoped to a Software Engineer candidate at Marsh & Mclennan.

Focus areas

  • Data Structures & Algorithms (DSA)
  • System Design (API Design)
  • Docker

The Marsh & Mclennan interview process

Marsh & Mclennan candidates report 5 rounds over 4-6 weeks. The stages below are what candidates describe, not a published process.

Automated Screening

Initial phase involving an online aptitude test, basic coding assessment, or AI-powered video interview to evaluate resume and foundational technical knowledge.

What to demonstrate

  • Initial phase involving an online aptitude test, basic coding assessment, or AI-powered video interview to evaluate resume and foundational technical knowledge
  • Depth in Data Structures & Algorithms (DSA)

How to prepare

  • Answer aloud and timed: Explain the lifecycle of an ASP.NET MVC request and how routing is handled.
  • Answer aloud and timed: How do you implement dependency injection in a.NET Core application, and what are its primary benefits?

Technical Rounds

Live coding sessions focusing on easy-to-medium level questions, SQL query design, and discussions about past projects and architectural choices.

What to demonstrate

  • Live coding sessions focusing on easy-to-medium level questions, SQL query design, and discussions about past projects and architectural choices
  • Depth in Data Structures & Algorithms (DSA)

How to prepare

  • Answer aloud and timed: Write a SQL query using complex joins and window functions to retrieve specific transactional data.
  • Answer aloud and timed: What is the difference between abstract classes and interfaces in C#, and when would you use each?

System Design Round

For mid-to-senior positions, candidates map out API communications and backend integrations.

What to demonstrate

  • For mid-to-senior positions, candidates map out API communications and backend integrations
  • Depth in Data Structures & Algorithms (DSA)

How to prepare

  • Answer aloud and timed: Explain how you would optimize a slow-performing database query or index in SQL Server.
  • Answer aloud and timed: Given an array of integers, write a function to find the two numbers that add up to a specific target.

Managerial and Behavioral Interviews

Panel interviews assessing alignment with corporate values, experience in Agile environments, and soft skills.

What to demonstrate

  • Panel interviews assessing alignment with corporate values, experience in Agile environments, and soft skills
  • Depth in Data Structures & Algorithms (DSA)

How to prepare

  • Prepare three examples from your own work, each with a decision you made and an outcome you can quantify.
  • Re-read the description of the managerial and behavioral interviews above and write down what you would ask to confirm before it.

Final HR Discussions

Final discussions with HR regarding salary negotiations and other employment terms.

What to demonstrate

  • Final discussions with HR regarding salary negotiations and other employment terms
  • Depth in Data Structures & Algorithms (DSA)

How to prepare

  • Answer aloud and timed: Write a clean, optimal algorithm to detect if a string is a palindrome while ignoring spaces and punctuation.
  • Answer aloud and timed: Design a RESTful API and explain how it would securely communicate with various backend services.

Where candidates lose points

Practice Structured Communication

When answering behavioral or technical design questions, use the STAR method (Situation, Task, Action, Result). This keeps your answers concise and ensures you highlight your specific contributions.

Brush Up on SQL Fundamentals

Do not neglect database questions. Candidates are frequently asked to write live SQL queries, explain indexing, or discuss database normalization during the technical rounds.

Understand Agile Ceremonies

Be ready to discuss how you actively contribute to sprint planning, standups, and retrospectives. Showing that you understand how to optimize these events to improve team velocity is highly valued.

Questions & practice

Given an array of integers, write a function to find the two numbers that add up to a specific target.

Medium · Coding & Algorithmic Problem Solving

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

How would you implement a function to reverse a linked list, and what is its time complexity?

Medium · Coding & Algorithmic Problem Solving

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Solve a logical puzzle involving probability or resource allocation to demonstrate your structured thinking.

Medium · Coding & Algorithmic Problem Solving

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Write a clean, optimal algorithm to detect if a string is a palindrome while ignoring spaces and punctuation.

Medium · Coding & Algorithmic Problem Solving

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Write a SQL query using complex joins and window functions to retrieve specific transactional data.

Medium · Technical & Domain Knowledge

Approach

  1. Name the grain you start from and join outward from it.
  2. Check whether any join is one-to-many before aggregating, or the sums inflate.
  3. Say which index the query would use, and what makes it unusable.
  4. Handle the rows that do not match: that is usually the actual question.

Follow-up

  • How does the query change if that join becomes one-to-many?
  • What happens to this when the table is ten times larger?

Explain how you would optimize a slow-performing database query or index in SQL Server.

Medium · Technical & Domain Knowledge

Approach

  1. Name the grain you start from and join outward from it.
  2. Check whether any join is one-to-many before aggregating, or the sums inflate.
  3. Say which index the query would use, and what makes it unusable.
  4. Handle the rows that do not match: that is usually the actual question.

Follow-up

  • How does the query change if that join becomes one-to-many?
  • What happens to this when the table is ten times larger?

Explain the lifecycle of an ASP.NET MVC request and how routing is handled.

Medium · Technical & Domain Knowledge

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

How do you implement dependency injection in a.NET Core application, and what are its primary benefits?

Medium · Technical & Domain Knowledge

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

What is the difference between abstract classes and interfaces in C#, and when would you use each?

Medium · Technical & Domain Knowledge

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

Design a RESTful API and explain how it would securely communicate with various backend services.

Medium · System Design & Architecture

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

How would you design a system to handle high-volume batch processing of financial transactions?

Medium · System Design & Architecture

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

Walk through the process of migrating a monolithic application to a microservices architecture.

Medium · System Design & Architecture

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

How do you ensure high availability and disaster recovery when deploying applications to the cloud?

Medium · System Design & Architecture

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

p99 jumped on one listing filter while p50 stayed flat

Medium · query planning · composite index · percentiles

After a release that added an owner_user_id filter to the resource listing, p99 rose from 90 ms to 1.9 s while p50 stayed at 40 ms. Traffic and row counts are unchanged. resource carries the index (tenant_id, status, updated_at DESC, resource_id DESC). The new query filters tenant_id and owner_user_id, orders by updated_at DESC, resource_id DESC, and takes 20 rows. On PostgreSQL, explain the shape of the regression, prove it from a query plan, and give the index you would add.

Approach

  1. Start from the shape. A flat p50 with a moved p99 means a subset of requests changed cost, not all of them, so the first job is naming the subset. Bucket the endpoint's latency by the tenant's row count; the natural hypothesis is that large tenants are a small share of requests and all of the tail.
  2. Get the plan for the new query on a large tenant with EXPLAIN (ANALYZE, BUFFERS). Expect an index scan over the tenant's range, a filter discarding most of it, then a Sort feeding the Limit, possibly reporting Sort Method: external merge Disk. Read actual rows on the scan node, not estimated.
  3. Explain why the existing index cannot serve it. A composite B-tree is seekable only as a left prefix, and with no equality predicate on status the scan cannot treat updated_at as an ordering, because rows in the tenant's range are ordered by status first. Everything matching must be read and sorted before LIMIT 20 can apply, so a tenant with 400,000 rows pays 400,000 rows to return 20.
  4. Add (tenant_id, owner_user_id, updated_at DESC, resource_id DESC). Equality on the first two columns leaves the index ordered by updated_at within that pair, so the plan becomes an index scan that stops after 20 rows with no Sort node. PostgreSQL can scan a B-tree backwards, so the DESC markers matter only if the two sort columns ever disagree in direction; keeping them explicit documents the order the keyset cursor depends on.

Follow-up

  • The endpoint paginates with OFFSET. What does page 500 cost with your index, and what does the keyset version cost?
  • How would you have caught this before release, given that a 10,000-row seed database produces the same plan shape at an unnoticeable cost?

Suggested practice checklist

Built from the rounds and topics Marsh & Mclennan candidates report.

Day 1 — Map the Marsh & Mclennan loop

  • Write out the reported sequence: Automated Screening, Technical Rounds, System Design Round, Managerial and Behavioral Interviews, Final HR Discussions.
  • For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 5 reported rounds, with the weakest marked.

Day 2 — Work Data Structures & Algorithms (DSA)

  • Spend the session on Data Structures & Algorithms (DSA), which Marsh & Mclennan candidates report being tested on.
  • Write one worked example in Data Structures & Algorithms (DSA) and time yourself on it.

Deliverable: One timed worked example in Data Structures & Algorithms (DSA).

Day 3 — Work System Design (API Design)

  • Spend the session on System Design (API Design), which Marsh & Mclennan candidates report being tested on.
  • Write one worked example in System Design (API Design) and time yourself on it.

Deliverable: One timed worked example in System Design (API Design).

Day 4 — Work Docker

  • Spend the session on Docker, which Marsh & Mclennan candidates report being tested on.
  • Write one worked example in Docker and time yourself on it.

Deliverable: One timed worked example in Docker.

Day 5 — Answer out loud: Technical & Domain Knowledge

  • Answer aloud, timed: Explain the lifecycle of an ASP.NET MVC request and how routing is handled.
  • Answer aloud, timed: How do you implement dependency injection in a.NET Core application, and what are its primary benefits?

Deliverable: Spoken answers to 2 reported Technical & Domain Knowledge question(s), under time.

Day 6 — Answer out loud: Coding & Algorithmic Problem Solving

  • Answer aloud, timed: Given an array of integers, write a function to find the two numbers that add up to a specific target.
  • Answer aloud, timed: How would you implement a function to reverse a linked list, and what is its time complexity?

Deliverable: Spoken answers to 2 reported Coding & Algorithmic Problem Solving question(s), under time.

Day 7 — Answer out loud: System Design & Architecture

  • Answer aloud, timed: Design a RESTful API and explain how it would securely communicate with various backend services.
  • Answer aloud, timed: How would you design a system to handle high-volume batch processing of financial transactions?

Deliverable: Spoken answers to 2 reported System Design & Architecture question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

Describe a time when you had to manage a technical conflict between two developers on your team.

Medium · Behavioral & Agile Competencies

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

How do you overcome challenges when managing or participating in Agile events like sprint planning or retrospe

Medium · Behavioral & Agile Competencies

How do you overcome challenges when managing or participating in Agile events like sprint planning or retrospectives?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Walk me through a highly complex project you recently completed, focusing on the hardest technical challenge y

Medium · Behavioral & Agile Competencies

Walk me through a highly complex project you recently completed, focusing on the hardest technical challenge you overcame.

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

How do you handle a situation where a project's requirements change mid-sprint?

Medium · Behavioral & Agile Competencies

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?