How can I use a 27-inch iMac (Late 2015) as a monitor for a Mac mini?

SCM17 wrote:

I would like to use a Mac Mini and the iMac as a monitor and no lnger use the iMac itself.

I feel it a waste of resources to get rid of my current iMac which is working great and upgrade without wasting it. It seems quite an ecological and economical solution.

In terms of an "ecological solution", consider this:

• Using that iMac as a monitor is likely to consume much more electricity than using a standalone monitor. (Plus, there is no way to use it as a hardware monitor. The best case scenario would be using it with Luna Display. In that case, it would be using electricity in addition to the electricity consumed by your main monitor – and there could be compromises in performance compared to using a standalone display.)
• Big Sur is enough to run current versions of many third-party browsers (such as Firefox and Brave), LibreOffice, the GNU Image Manipulation Program (GIMP), and the commercial Affinity V2 applications. And that Mac has a very nice 27" 5K screen, expandable RAM, and USB 3.0 ports to which a SSD could be attached. While it can't run current versions of macOS, it might still make a very nice machine for someone who wanted to use it for the aforementioned applications. That could extend its useful life.
• If you just wanted to get rid of it, Apple might not offer much (if anything) in trade-in for it – but they would take the computer for recycling for free. Sending it to recycling would be better than putting it into a landfill.

The 2015 iMac is an all-in-one computer. It does not support cabled video input for use as a standalone display.

The new Mac mini can only use the 2015 iMac as a secondary networked display with limited function.

If you get a new Mac mini, then you will need to use an external display as the primary display.

see > Connect a display to Mac mini - Apple Support

Target Display Mode went away when Apple introduced the first 27" 5K Retina iMac in Late 2014. Your iMac does not have it. Later, Apple added restrictions on newer Macs and versions of macOS. These restrictions mean that none of the current Macs support using ANY iMac as a Target Display.

Your iMac is also too old to be an AirPlay Receiver for AirPlay to Mac, a lower-quality way of using a Mac''s screen as an extra display. There are third-party AirPlay applications that might let you use that iMac as a second screen. These would probably be limited to AirPlay v1, and a maximum resolution of 1920x1080, and an AirPlay connection would also have a potential for lags and artifacts. You would still need at least one real hardware monitor (or TV) to be able to set up AirPlay connections. All in all, that sounds like more trouble than it is worth.

There is a third-party workaround product called Luna Display that might let you use that 27" 5K iMac screen as a second screen.

Astropad – Blog – How to Use Your iMac as a Monitor: A Step-by-Step Guide [2025]

"Like any product, Luna Display does have limitations and it’s not recommended for high-motion content. For example, I wouldn’t recommend using Luna Display for gaming or video editing.

Since Luna Display is squeezing a 5K display through a USB cable, when lots of motion occurs on-screen (like from video playback or moving a window quickly) the screen can temporarily become pixelated before it renders in at full resolution. "

27" 5K monitors which have the pixel density of your iMac are still relatively few in number and expensive. Apple wants $1599+ USD for theirs. For a while, only LG made a competing 27" 5K display. That's changed, and now there also are, or soon will be, 27" 5K (5120x2880 pixel) displays from Samsung, BenQ, Asus, and ViewSonic.

My advice would be that if you are interested in exploring the possibility of using Luna Display to salvage that nice 27" 5K screen, you still should pick out a good standalone hardware display for your Mac mini. Although large 4K displays aren't quite as nice as large 5K ones, there are more 27" – 32" 4K displays than you can shake a stick at, and some decent basic ones are fairly cheap ($300 – $400 USD, meaning you could potentially buy three 27" 4K displays to "max out" a new Mini for less than the cost of one 27" Apple 5K display).

not as the only display (their old target display mode is long gone), and it would use lots more power than a normal monitor

Use iMac as monitor: 5 Solutions - 9to5Mac

You cannot without function loss. Just use your old iMac as needed and get a new dedicated monitor for your new Mac Mini. That is what I did as my older iMac still works fine.