Did this round about six weeks ago for a senior SWE role (they call it L4/L5 equivalent, roughly). Going to brain-dump because I wish I'd found something like this before.
First: Mercury is a fintech, and the system design round leans hard into that. Don't expect a generic "design Twitter" prompt. My prompt was essentially: design a money movement system that handles ACH, wire transfers, and internal ledger entries while keeping accounts eventually consistent. Yeah. Welcome to fintech.
The interviewer was sharp. Not adversarial, but they pushed on every assumption. When I said I'd use an event-driven architecture with Kafka, they asked how I'd handle duplicate events. When I said idempotency keys, they dug into how you'd generate those at scale. The whole thing felt like a conversation with someone who has actually built this stuff, not someone checking a rubric.
What they seemed to care about most: Failure handling. What happens if a Kafka consumer crashes mid-processing? How do you avoid double-debiting an account? Consistency vs. availability tradeoffs. They weren't looking for a single answer, they wanted you to articulate the cost of each choice. Observability. I mentioned metrics + alerting pretty late and the interviewer immediately asked why I hadn't brought it up earlier. Fair point. Regulatory/compliance surface area. They didn't expect me to know banking law, but they wanted me to acknowledge that certain data needs an audit trail.
Structure-wise: 10-15 min requirements clarification, then I drove the design, then they started poking holes. Total ~60 min.
I came in with big-tech distributed systems background and I think that actually helped. But you need to translate that knowledge toward financial correctness: idempotency, ledger double-entry patterns, regulatory holds. If you've only ever designed content feeds, spend some time reading about distributed ledger patterns before this round.
Got through to final round. Didn't get the offer in the end (leveling disagreement, I think), but the design loop itself was one of the better ones I've done this cycle.