Can I use an iMac as a Mac mini's screen?

Hello, am graven.

Here are the rules for Target Display mode on iMac...

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

Target display mode is a feature that was available only with certain older iMac models.

Only the following older iMac models can be used as an external display when plugged into another older Mac model:

• iMac models introduced in 2011, 2012, 2013, and mid 2014 using Thunderbolt or Thunderbolt 2 cable.
• 24-inch and 27-inch iMac models introduced in 2009 and 2010 using Mini DisplayPort cable.
• The intended target iMac must be running macOS 10.13 High Sierra or earlier; it does not work with macOS 10.14 Mojave or later, or with Boot Camp and Windows.*
• Retina display iMacs won’t work as target displays.

• In all cases the source Mac that you're connecting FROM must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.

Maybe we can figure out why your iMac is slowing down. It could be anything from a simple malware installation to a hardware failure. Some things are fairly easy to remedy.

Start with an EtreCheck evaluation of your Mac.

EtreCheck is a diagnostic app that gathers and reports important debugging information about your Macintosh. It is very highly regarded and meant to be used with Apple Support Communities to help others help you troubleshoot and resolve your Mac problems.

Please navigate to EtreCheck.com and download the free version of EtreCheck. Once you’ve run the app and created your report please post it with your reply to this message. 

You must upload the full report. For instructions on how to attach your EtreCheck report to your reply, please click  How to use Add Text when posting…

We can use the EtreCheck report to look for the more obvious things that may be causing your problem and advise how to correct them.

Yes, the hardware is obsolete.

The HDD is slow, The RAM is low.

And you have CleanMyMac installed - that's not doing you any favors.

I don't think I'd put any money into this machine.

But if you want to upgrade, then back to your original question.

Yes, you can use this iMac as a second screen for a more up-to-date Mac mini, IF that mini meets this requirement:

• ... the source Mac mini that you're connecting FROM must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.

So find a quality used mini and you're in business. 🙂