Use your iMac as a display with Target Display Mode
Target Display Mode lets you use the built-in display on a supported iMac as an external display for another Mac.
To use your iMac as a display in Target Display Mode, connect it to another Mac using the correct cable (see System Requirements below). Then press Command-F2 on the keyboard connected to your iMac. When you're done, press Command-F2 again.
System Requirements
You need the following to use Target Display Mode:
A Mac notebook or desktop with a Mini DisplayPort or Thunderbolt port. If your Mac has Thunderbolt 3 (USB-C) ports, you can connect it using the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
A Thunderbolt cable or a Mini DisplayPort to Mini DisplayPort cable
A supported iMac with OS X v10.6.1 or later
See the list below for iMac models that support Target Display Mode, the type of cable you need to connect that model of iMac, and the kind of port you need on your other Mac.
The following older iMac models can be used as an external display when plugged into another older Mac model.
iMac models introduced in 2011, 2012, 2013, and mid 2014
To use any of these iMac models as an external display:
The iMac used as a display must have macOS High Sierra or earlier installed.
The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
The cable connecting the two Mac computers must be a Thunderbolt or Thunderbolt 2 cable.
24-inch and 27-inch iMac models introduced in 2009 and 2010
To use any of these iMac models as an external display:
The iMac used as a display must have macOS High Sierra or earlier installed.
The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
The cable connecting the two Mac computers must be a Mini DisplayPort cable.
