Can I use an older iMac computer as a monitor for a 2023 Mac mini? If so which year iMac would work ?

No because the 2023 Mac mini can not use any iMac as a display in Target Display Mode.

carefully study > Use your iMac as a display with target display mode - Apple Support

Hi, DRMJP, yes, it is Technically possible, just, as den.thed mentioned, not in Target Mode, you’d have to look into getting DuetDisplay/LunaDisplay software, which isn’t free… so, not really that practical, yes, you could in theory, do it, but you’d have to spend some extra money to do so… it’s your call, in the end… IF you had a spare working 27” iMac and wanted to give it a go vs. Buying a new( er) 27-32” monitor, which isn’t super expensive… even going for “open box” or refurbished special deals…

John B

Hello, I know this is probably a bit late for you but it still comes up at the top of google results. There are many free softwares you can use to run your imac as a remote desktop over the "internet" (or right next to your mac mini on your home network). The two I have used to do this are Google Chrome Remote Desktop, or Parsec. Both of these are free and very user friendly, and can be used from mac or windows desktops. They will use a bit of your desktops power to run, so not ideal if you are running heavy programs, but great if you are just browsing the internet or doing word/office work.

It is much easier to set this up as a second monitor (since you can use the first monitor to set everything up), but you can get a cheap $100 monitor for the side and use the imac as your main display. I recommend Parsec because Colors come accross very well and video playback is very smooth, but chrome remote desktop is a bit more reliable for slow connections. (Both will work best with a wired ethernet connection, as wifi connections will be more unstable).

More work than a lot of people are up for, but its free and you get to use that great Imac screen for a bit longer. Added bonus, the imac is still running as a computer, so if you have any side jobs to run you can do them on there, and keep your mini free for your main tasks.

you’re welcome, yeah, not really worth it, but it is “technically” possible if not practical, so just go with an IPS monitor and you should be OK, you could erase and sell off your 2015 iMac 27”

John B