Want to use an iMac as an external monitor? You need an old version of macOS

One of the most common reader questions we get is, “Can I use my iMac as an external monitor?” We previously provided an almost-comprehensive answer to the question before Apple restored a page on its site that offered technical details. The general answer is that only iMacs models shipped from 2009 to mid-2014 (the seventh- and eighth-generation models) had the right graphics circuitry for this Target Display Mode feature.

Since then, many people have asked about what seemed to them a bizarre, unnecessary, and inaccurate requirement—that is sadly absolutely true. Does an iMac have to be running an old version of macOS as well to support Target Display Mode? Yes, it must. And so must the Mac connecting to it!

Apple eliminated Target Display Mode support from macOS 10.14 Mojave. In order to use your iMac as a display, it has to be both the correct model year and have macOS 10.13 High Sierra or an earlier version of macOS (or Mac OS X) installed.

The Mac that uses the display of an iMac of the correct vintage with the correct macOS version installed has its own requirements: the controlling Mac must be a model released in 2019 or earlier and must have macOS 10.15 Catalina or earlier installed. macOS 11 Big Sur apparently modified the capability for Intel Macs to target an otherwise qualifying iMac as a display, and the Apple silicon M1 Macs can only run Big Sur—or, soon, Monterey.

Update: A few readers with Big Sur installed have written in to say that they can, in fact, still control an iMac with their Big Sur Mac. 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.) Apple only supports Catalina and earlier with this feature, which means that the company could fully disable it in an update to Big Sur without contradicting its support documents.

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

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