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

A limited range of iMacs from 2009 to 2014 can be used as external monitors, but only when running certain versions of macOS.

previously provided an almost-comprehensive answer, read our super FAQ

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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 removed 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.

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

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We’ve compiled a list of the questions we get asked most frequently along with answers and links to columns: read our super FAQ to see if your question is covered. If not, we’re always looking for new problems to solve! Email yours to mac911@macworld.com including screen captures as appropriate, and whether you want your full name used. Not every question will be answered, we don’t reply to email, and we cannot provide direct troubleshooting advice.

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