Will my 2014 Mac Mini boot an SSD that already has Linux on it? I have an SSD from another computer that has Ubuntu Linux installed on it and fully working. It's a standard UEFI Ubuntu install using the GRUB bootloader, and the entire SSD is used (i.e. there are no additional partitions).

If I put this SSD into my 2014 Mac Mini, will it successfully boot?

I can put the SSD in most other semi-modern computers and it boots fine. For example I have done this in a Lenovo Thinkpad, Windows gaming PC, etc.

Just to clarify I'm not talking about installing Linux or reinstalling macOS - I'm talking about booting the Linux OS that's already installed on the SSD.

If the SSD will not boot, what do I need to do to make it boot?
There is no way to know in advance if you can boot the operating system on this SDD. The the only way to know for sure would be to try booting. The Mac Startup Manger is employed to either manually or automatically determine what code needs to be loaded in to RAM memory and executed in order to boot the computer. This Startup Manager does not recognize all the possible valid boot configurations and therefore may ignore the Grub and/or Ubuntu startup files stored on the SSD. One way to avoid this problem would be to first boot the a rEFInd USB flash drive image file. A link to download this file can be found at the Getting rEFInd from Sourceforge website. If rEFInd is necessary to boot Ubuntu, then you can either modify the name and location of the startup files on the SSD to avoid using rEFInd or install rEFInd on to the SSD.