Q: Was in a rear-end collision that affected my brakes. Mechanic says master cylinder is the issue. Could this be do to the wreck

To determine if crash forces led to master cylinder failure you would have to determine exactly "what" failed within the master cylinder. If the master cylinder bore is scored or worn, for example, thus causing significant output pressure loss, that would not be crash related but rather is a consequence of long standing wear. Unless the mechanic has already explained exactly how he concluded the master cylinder is faulty, that is explained specific pressure tests that were performed and what the results were, I wouldn’t be too quick to conclude that that is even the problem area in the braking system. To address your other concern, damage to the brake master cylinder due to crash forces of almost all types is implausible and thus undoubtedly unusual. In your circumstance, I would first seek clarification from the mechanic who "tested" the master cylinder. Alternatively, if you want a certified mechanic from YourMechanic to get to the bottom of this, just request a brakes, steering and suspension inspection and the responding certified mechanic will look at what you have in person and assist you with a resolution. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.