Use an old iMac as the monitor for another Mac

If you have an old Mac, typically an unused iMac with its large built-in monitor, you might be wondering if you can use it as a display for a newer Mac. Wouldn’t it be great if you could plug your new Mac laptop into an old iMac and use it as a giant monitor.

The good news is that you can. And it’s pretty easy these days to use an iMac as a monitor for another Mac. In the old days you used to have to either physically hack the old Mac, or use some networking software, but Apple has taken matters into its own hands with a great feature called Target Display Mode.

If you are wanting to use a second display with your Mac and not have your Mac’s screen on read: How to turn a Mac’s screen off.

Get to know Target Display Mode

Target Display Mode (TDM) is a relatively new feature introduced by Apple. With TDM you can connect two Mac’s together and use one as a second display. You may heard of Target Disk Mode, it’s older cousin that enables you to use one Mac’s hard drive as an external drive on an old Mac. Target Display Mode is a similar feature.

How do I turn on Target Display Mode

Press Command-F2 to turn on Target Display Mode

It’s easy to use Target Display Mode. All you need is an iMac that supports TDM and a Thunderbolt cable. Here is how to set up TDM on both Mac’s:

Connect both computers using a Thunderbolt cable
Make sure both computers are switched on and awake
Press Command-F2 on the keyboard of the iMac (the computer being used as the display) to enable Target Display Mode.

You should now be viewing the screen from the other Mac on the display of the iMac. When you are finished press Command-F2 on the keyboard of the Mac being used as a display to exit Target Display Mode.

If in Keyboard System Preferences you have selected “Use all F1, F2 etc. keys as standard functions keys,” then you need to use Command-Fn-F2 instead. Apple suggests that you use an Apple keyboard (the modern aluminium type) with the special keys if you are finding it now working.

Target Display Mode system requirements

Here are the iMac computers that support TDM:

iMac (27-inch late 2009)
iMac (27-inch Mid 2010)
iMac (Mid 2011)
iMac (Midi 2012 and later)
Note! The new iMac with 5K Retina Display is not compatible with TDM.

Target Display Mode requires both Mac’s to feature MiniDisplayPort or Thunderbolt. You also need to have a specific model of iMac. On some of the older iMac’s you connect Mini DisplayPort to Mini DisplayPort, or Mini DisplayPort to Thunderbolt and on the newer Macs it uses a ThunderBolt to Thunderbolt connection.

Confused? Apple has a chart on this its Target Display Mode support page that lists all the Macs that support Target Display Model along with answers to other questions you might have.