One of the most common questions we get from readers is, “Can I use my iMac as an external monitor?” We previously provided a nearly complete answer to the question before Apple restored a page on their site that offered technical details. The general answer is that only the iMacs shipped from 2009 to mid-2014 (the 7th and 8th generation models) had the proper graphics circuitry for this target display mode feature.
Since then, many people have asked about what they found to be a strange, unnecessary, and inaccurate requirement; that is, sadly, absolutely true. Does an iMac need to be running an older version of macOS to support target display mode? Yes, you must. Y So must the Mac that connects to it!
Apple removed support for target display mode from macOS 10.14 Mojave. To use your iMac as a display, it must be the correct model year other have installed macOS 10.13 High Sierra or an earlier version of macOS (or Mac OS X).
The Mac using the correct vintage iMac display with the correct version of macOS installed has its own requirements: the controller Mac must be a model released in 2019 or earlier and it must have macOS 10.15 Catalina or earlier installed. macOS 11 Big Sur apparently removed the ability for Intel Macs to target an iMac rated as a display, and silicon Apple M1 Macs can only run Big Sur or, soon, Monterey.
This Mac 911 article is an answer to a question posted by Macworld reader Rick.
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