I've just come out the other end of the MongoDB interview process, and I wanted to share how I got ready, since other people's write-ups helped me so much.
I'm a backend engineer by trade, with time at a large tech company and at a startup. Over the course of this job hunt I had a handful of interviews in flight simultaneously, yet MongoDB topped my wish list, and that's where I landed an offer. My pay is rising by around 75%, which makes all the effort feel justified.
The first step was a phone screen with LeetCode-style questions plus behavioral ones. After that, the onsite had five separate rounds: a coding round, system design, a code review round, a conversation with the hiring manager, and one with a director. I prepared for every round on its own terms instead of vaguely telling myself I was studying for interviews. Mock interviews made the biggest difference, for both coding and system design. Reading only goes so far; the only way to learn whether you're truly prepared is to think aloud against the clock.
For system design, I was handed a question with a fintech flavor, involving real-time transactions or trading. Having practiced system design in advance was a real help in this round.
Plenty of candidates neglect the code review round. I put in time on catching concurrency bugs, idempotency gaps and correctness flaws, and on stating clearly what I'd noticed rather than offering a fuzzy "something seems wrong here."
The behavioral portions, which covered the phone screen, the hiring manager and the director, are organized around MongoDB's company values. I prepared stories drawn from real incidents and launches I'd worked on, and I planned beforehand which value each story illustrated, so nothing was improvised on the spot.
In coding, the best investment was working through Blind 75 thoroughly, aiming to understand the underlying pattern of each problem instead of memorizing its solution. I also revisited some MIT algorithms lecture videos to firm up the basics, which made the patterns make sense rather than feel like rote steps. I added mocks on top to practice identifying which pattern applied after the interviewer layered on new constraints, instead of simply matching against problems I'd seen before.
Overall the process was rigorous but plain: a phone screen, then a five-round onsite, with no surprises in structure. Code review was the round I underrated the most beforehand.
My advice if you're getting ready for this: find out which team you're meeting and prepare the kinds of systems that team would genuinely build, rather than generic system design. Do mock interviews, not just reading. If your fundamentals are shaky, the MIT algorithms lectures are worth your time. And don't brush off code review; it's easy to neglect because it lacks the obvious weight of coding or system design.
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