Can you use your iMac as a monitor?

Answer 1

Just to add to the previous answer, you can only use an iMac as an external monitor for models created in 2014 or earlier. Apple took away this function, called Target Display Mode in iMacs built after 2014.

Here is an Apple Support page which explains how to used Target Display Mode: https://support.apple.com/en-us/HT204592

Answer 2

IF you are connected to another Mac. Then the answer is usually yes. Use a double ended Thunderbolt cable. Plug into both systems when they are off. Start the NON-iMAC unit first. Then start the iMac. It may or may not jump over. If not them press Command - F2 in the NON-iMac. The iMac should then come online. Sometimes you need to unplug / replug the iMac for the other unit to recgnize it. Note that you will have to create all new color profiles for this setup. And you need to take your time calibrating.