Connect iMac to iMac to use as monitor

Your 2013 iMac is Thunderbolt, so the connection has to be Thunderbolt all the way. A Mini DisplayPort cable won't work.

Connect it using the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter and a Thunderbolt cable.

Greetings uwam723,

Thanks for posting your question on the Apple Support Communities. Based on the information you've provided I believe that Apple Article - HT204592 will be beneficial. On this article you will be able to review the troubleshooting steps if Command-F2 isn't working.

Hi there,

Thunderbolt and Mini DisplayPort use the same port and therefore are the same sizes. You can plug a Thunderbolt Cable or Adapter into the Mini DisplayPort. Based on https://support.apple.com/kb/sp687?locale=en_US and https://support.apple.com/kb/sp688?locale=en_US both the 21.5 inch and 27 inch iMacs from Late 2013 have Thunderbolt Ports.

iMac (21.5-inch, Late 2013) - Technical Specifications

iMac (27-inch, Late 2013) - Technical Specifications

"Two Thunderbolt ports"

Thunderbolt and Mini DisplayPorts look the same but Thunderbolt ports will have a lightning bolt icon next to them.

Thunderbolt 2 and Mini DisplayPort plugs are the same size. Thunderbolt 3 and USB-C plugs are the same size.

Thunderbolt plugs will have lightning bolt icons.

Greetings uwam723,

Here are the two products you'll need:

https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter

https://www.apple.com/shop/product/MD861LL/A/apple-thunderbolt-cable-20-m (You can also get a longer cable)