What You Need to Use Your iMac as a Monitor

Display mode-compatible 27-inch iMacs have either a bi-directional Mini DisplayPort or a Thunderbolt port (depending on the model). You need the proper ports and cables to make the connection between the two Macs. The iMac used as a display must be running macOS High Sierra (10.13.6) or earlier. The source Mac must have macOS Catalina (10.15) or earlier installed.

Compatible iMacs

The iMac no longer supports Target Display Mode, but several models sold in late 2009 through 2014 support the feature.
 

How to Enable and Disable Target Display Mode

If you have a suitable iMac model, appropriate cables, and the specified operating system, you can set up Target Display Mode. Here's how:

Turn on the iMac that will be used as the display and the Mac that will be the source.

1.   Connect the two Macs with either a Mini DisplayPort cable or a Thunderbolt cable.

2.   The connection typically happens automatically, but if your iMac doesn't automatically enter Target Display Mode, press Command+F2 on the iMac to manually enter Target Display Mode. The iMac displays the screen contents of the source Mac.

3.   When you are ready to exit Target Display Mode, manually turn off the feature by pressing the Command+F2 keyboard combination or by disconnecting the iMac from the source Mac.

What to Do If Target Display Mode Doesn't Work

If Target Display Mode isn't functioning as expected, try these things to help make the connection.

 Use Command+Fn+F2. This may work for some keyboard types.

1.   Make sure the Mini DisplayPort or Thunderbolt cable is connected correctly.

2.   If the iMac being used as a display is currently booted from a Windows volume, restart it from the normal Mac startup drive.

3.   If you're currently logged in to the iMac you intend to use as a display, log out and return to the normal login screen.

4.   A few third-party keyboards don't send the Command+F2 correctly. Use another keyboard or the original keyboard that came with your Mac.

Things to Consider

• The iMac will continue to run its operating system and any applications that were open when it entered Target Display Mode.
• While in Target Display Mode, only the keyboard's display brightness, volume, and Target Display Mode key combinations are active. Any other keyboard input is ignored. USB, FireWire, and inputs other than the keyboard are also ignored.
• The source Mac can't make use of any of the display iMac's features, including the built-in iSight camera.
• Target Display Mode can be helpful in many situations, but it's not a full-time substitute for having a dedicated display for another Mac.

 Should You Use Your iMac as a Display?

If a temporary need arises, sure, why not? But in the long run, it just doesn't make sense to waste the computing power of an iMac, nor does it make sense to pay for the energy the iMac needs to run when you're only using the display. Remember, the rest of the iMac is still running behind the scenes, consuming electricity and generating heat.

 If you need a large display for your Mac, grab a decent 27-inch or larger computer monitor. It doesn't need to be a Thunderbolt display; almost any monitor with a DisplayPort or Mini DisplayPort will work well.