Downgrade 2018 Mac Mini to High Sierra?

It depends. No Mac can run an OS that pre-dates the one it shipped with.

If it shipped with High Sierra and you want to downgrade from Mojave use your Time Machine backup to revert:

Restore your Mac from a backup - Apple Support

Does that mean I can run the Mini with high Sierra or not? I need CUDA and that only works on high Sierra. So it would be good to know if it runs with high Sierra. Unfortunately I am not a native speaker and I did not understand the answer correctly.

Thanks for the answer. Too bad that it does not work with the new one. Then I hope that Apple and Nvidia make it possible again to run cuda per eGpu under Mojave. At the moment it does not look good. Hope for the Mac Pro 2019.

The 2018 MacMini can not possibly run High Sierra natively because

the new hardware on the Mini is totally unsupported.

The only alternative would be to get something like VMware Fusion,

Parallels Desktop Mac, or Virtual Box and run the earlier macOS

as a virtual machine. Depending on the nature of the apps required,

the performance may be more than acceptable.

FWIW, you can download Parallels Lite from the MacApp Store

which is free for installing any macOS, OSX, or Linux virtual

machine (Windows requires a subscription fee).

I've been struggling with this for over 2 weeks! After creating several bootable external drives and following all the steps laid out to put a new (in this case, older) OS on my new mac mini...no luck. I even ordered a cheap usb keyboard because I thought it had to do with the bluetooth keyboards not talking to the mac mini during boot up. Still no luck. What happens to me is that I got to the screen that lets you choose between the Mojave and High Sierra OS. Then I'd choose High Sierra and eventually...after 1-2 minutes there was just a big (/) symbol on the screen and the mac mini turned off. So, sadly for me, and not because of Apple - I can't use my new mac mini until Pro Tools is compatible with Mojave. But, I still don't understand why a new mac isn't backwards compatible with older OS. Oh well...

An older OS does not have the drivers for the newer hardware. That's why you're prohibited from using them.

Apple has always done this. With very few exceptions, a Mac will not run an OS that is older than what it shipped with. My 2010 Mac Pro came with 10.6.4. It will not boot to the 10.6.3 retail disk for the same reason. The shipped OS has hardware drivers on it the retail disk does not.

+1 for woodmeister50's comment.

As an additional item, VirtualBox 6.0 has recently been released. VirtualBox has historically been truly awful for running Mac guest operating systems (virtual machines), however version 6.0 finally adds some Mac guest addition support drivers which hopefully make this a big improvement over previous versions for running Mac guests.

This previous lack of Mac guest additions has meant it has in the past been unable to support multiple CPU cores for a Mac guest, or as standard better video resolution support, or shared folder support and so on. I have not been able to find a list of what guest features these preliminary Mac guest additions support. If anyone knows adding a reply here with that info would be helpful.

Why would you want to downgrade?

Maybe if you provide the reason you are thinking of downgrade, we might be able to offer and alternative that is simpler to accomplish.

It all depends on exactly which model of Mac mini you have.

If it is old enough then it can be downgraded.

If it is the brand new Mac Mini that Apple just release then no.