The 27-inch iMacs introduced in late 2009 included the first version of Target Display Mode, a feature that allowed iMacs to be used as displays for other Apple devices. However, by the time Apple released the 2014-2015 Retina Display iMacs, they no longer supported the feature.
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.
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.
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.
Using Multiple iMacs as Displays
It’s possible to use more than one iMac as a display provided all the Macs—both the iMacs used for display and the source Mac—are using Thunderbolt connectivity. Each iMac used as a display counts against the simultaneously connected displays supported by the Mac you’re using as the source.
