To provide you with the best experience, Office for Mac supports Apple’s three most recent versions of macOS. With the release of macOS 10.15 Catalina, Microsoft 365 for Mac and Office 2019 for Mac support macOS 10.15, 10.14, and 10.13. Moving forward, as new major versions of macOS are made generally available, Microsoft will drop support for the oldest version and support the newest and previous two versions of macOS.
What if I don't update macOS to a supported version?
If you’re on an unsupported version of macOS, your Office apps will still work but you would no longer receive any updates including security updates. For example, if you’re on macOS 10.13, but later you don’t update your macOS to a supported version when macOS 10.13 is no longer supported, you won’t receive any updates of any kind.
As new major versions of macOS are released and older versions become unsupported, we highly recommend you continue to update your macOS to a supported version in order to receive mainstream support such as security updates as well as new features (if you’re an Microsoft 365 for Mac subscriber). This applies for both Microsoft 365 for Mac and Office 2019 for Mac users.