How to use an iMac as an external monitor

Many readers want to use an old iMac as an external monitor—it’s one of the most frequent Mac 911 questions! We previously provided an almost-comprehensive answer to the question before Apple restored a page on its site that offered full technical details after deleting it for a period of time.

How to use a new iMac as an extended display

If you are lucky enough to have one of the following iMacs that you can use as an extended display you can use Universal Control to do so.

Compatible Macs include:

• 21.5in iMac (2017 and later)
• 27in iMac (2015 and later)
• 24in iMac
• iMac Pro

Want to use your iMac as an extra display here’s what to do:

1. Open System Preferences/System Settings on your Mac and select Displays.
2. Click on Add Display. You’ll see the names of your secondary Mac or iPad .
3. Select your iPad or the secondary Mac.
4. If the display settings don’t suit you (the test on our displays went really small) click on Display Settings and select the monitor that you want to use as the default in the column on the left.
5. If you find that your displays are mirrored and would prefer the displays to be extended, click on the drop down box beside Use as: and rather than Main display choose Stop mirroring.
6. If you find the display on your secondary Mac is the wrong resolution select Scaled and then choose an option that seems best.

This is a little like the way Sidecar works when you share an iPad screen with a Mac. The display of your primary Mac is extended onto the display of your secondary Mac – you can’t access your secondary Mac.

How to use an old iMac as an extended display

If you are hoping that it will be a way to use an old iMac display with your new Mac then be prepared to be disappointed – the feature most likely won’t be compatible unless your iMac is one of those mentioned in the compatibility list above (2015 or 2017 and later depending on the size). 

The general answer is often disappointing: only iMacs models shipped from 2009 to mid-2014 (the seventh- and eighth-generation releases) had the right graphics circuitry for what Apple called Target Display Mode.

But Apple imposed a second software requirement starting with the release of macOS 10.15 Catalina. Despite the many people who have written in to ask if it’s true, reality won’t budge. To use Target Display Mode, both the old iMac and the Mac you want to connect to it must be running outdated releases of macOS:

• iMac with Target Display Mode: This Mac must have macOS 10.13 High Sierra or an earlier version of macOS (or Mac OS X) installed.
• Mac using iMac as a display: Your controlling Mac must have macOS 10.15 Catalina or earlier installed.

We don’t know precisely what happened. Did Apple remove code in 10.14 Mojave that allowed an iMac to activate Target Display Mode? Did the company shed the instructions in macOS 11 Big Sur that still supported sending video an iMac in the correct manner? Apple doesn’t reveal these sorts of details.

Some readers have found that with Big Sur installed on the controlling Mac they could, in fact, still drive an iMac’s display. However, we’ve also heard from other readers who were frustrated they could not. One reader said that it only worked after pressing Command-F2 repeatedly on the iMac, sometimes a few seconds apart, which may have overcome the iMac’s resistance—how, again, we don’t know, since it’s not supported.

This Mac 911 article is in response to a question submitted by Macworld reader Rick.

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