Troubleshooting tips for using an old iMac as an external display for a Thunderbolt 3 Mac

Because of the decreasing value of older computers and sometimes children leaving the nest and their old computers behind, you could wind up with an older iMac that no longer meets your day-to-day needs, but which you could transform into a monitor.

All the iMacs models that can work in what Apple called Target Display Mode as a secondary display for a modern Mac require an adapter to connect a Thunderbolt 3 port on the iMac to a Mini DisplayPort or Thunderbolt 2 port on the older iMac, and have it function as expected. (Apple appears to have recently removed all mention of Target Display Mode from its site: its former FAQ redirects to an introduction to using Catalina.)

iMacs released from late 2009 through mid-2014 can be used as external displays. Those that followed cannot, starting with the 27-inch 5K late-2014 edition.

Adapters

For the 2009 and 2010 iMac models with Mini DisplayPort, purchase a USB-C to Mini DisplayPort adapter. Fortunately, because the iMac has a jack into which you plug a cable, this is not nearly as difficult as driving an Apple display that only has a plug on the end.

A number of sub-$20 Mini DisplayPort to USB-C adapters are readily available, such as this model from Allsmartlife. Some USB-C docks also include Mini DisplayPort support. You also need a Mini DisplayPort to Mini DisplayPort cable, which costs under $10.

For the 2011 to 2014 Thunderbolt 2 iMacs, you need Apple’s or a third-party Thunderbolt 2 to Thunderbolt 3 adapter (Apple’s is $49), and a Thunderbolt 2 to Thunderbolt 2 cable, which can cost $20 to 30. Even though Thunderbolt 2 and Mini DisplayPort connectors appear identical, the wiring is different and a different cable is required. Make sure you are purchasing a cable with a lightning-bolt icon on both ends that’s has a rectangular connector, not the rounded USB-C one, which is used for Thunderbolt 3.

Making the conection

With the right cable in hand and your two machines connected, started up, and past the FileVault login screen (if enabled), your Thunderbolt 3-equipped Mac should simply recognize the iMac as a target display and the iMac’s speakers should appear in the Sound preference pane as an audio target. Other interface ports and the iMac’s camera aren’t available to use.

If that doesn’t occur, press Command-F2 on the keyboard connected to the iMac you want to use as a display. On a laptop or a keyboard that doubles function and other keys, press Command-Function-F2.