Does Catalina support Target Display mode?

Macbook Pro - 2017 Touchbar Catalina V10.15 - Connected to....

Display iMac - Late 2013 27" Catalina V10.15

Using Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter

I have been speaking to apple support both in store and on the phone, not very helpful over the phone especially giving steps on SMC... Holding Shift+Ctrl+Option 10-30sec safe mode etc. Even with SMC reset, CMD+F2 didn't help. I'm contemplating formating my iMac and downgrading back to Mojave to see if it helped. Previously it worked fine

Which versions of macOS broke Target Display Mode? Whether TDM is supported or not depends on the models of Macs involved, not the OS.

Yes! iMac sound works fine

I'm pretty new to all this, so maybe stating the obvious, so apologies if so, but anyway... when I first tried to set it up I read you just had to press cmd+F2 but later discovered that this only works at the log in screen and not once you've logged in.

Also it only seems to work using my official Mac keyboard and not my Logitech one.

No, I am not really interested in using an iPad as a second display, as it is too small. My set up is as follows...

Main Mac - 2017 21.5" Catalina V10.15 - Connected to....

Display Mac - Mid 2011 21.5" High Sierra V10.13.6

Using Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter

This has always worked up until the Catalina upgrade. The main Mac user logged in, the display mac user logged in - press cmd f2, wait a second or 2 and I have an extended display. Now nothing, not even a flicker.

Interesting - It has always worked with both mac users logged in, I don't think I ever tried it at the login screen. I also note the comment about the keyboard. My Wired keyboard to the 2017 iMac ceased working a short while ago, the only way I could get it to work was plugging it in to a USB extension lead, all was then good.

I'll do some further testing with and without the USB extension, and whether being logged in or not makes any difference. I'll report back my findings.

Ok, After a bit of testing - Keyboard still does not work without the extension lead (annoying but I can live with it) tried all scenarios still no go. Tried the same set up using my wife's MacBook Air running Mojave still nothing.

Then it dawned on me - You use the target display keyboard to press cmd f2 NOT the main display keyboard.

Stupid Me!!

Works a treat.

Sorry for wasting all your time.but thank you for contributing

Hi drshwartz

good day there ,I have just refresh my iMAC 2013 to Catanila and I trying to using TDM with my MacBook and Lenovo , neither of them are working .

i have THB connectors . Still not work , could you please kindly let me know how should O do to make it work

Two things that I noted in the article you linked:

a) It only mentions iMac models up to 2015, which suggests the original text is rather old. It says the article was "updated" in November 2019, but it is unclear what was updated. There is no mention of Catalina.

b) There is one incorrection in the article. It says

"Your iMac should automatically recognize the presence of a digital video signal at the Mini DisplayPort or Thunderbolt port and enter Target Display Mode."

This is (fortunately!) not true. You have to explicitly press Command-F2.

I always have the Thunderbolt cable connected between the MBP and iMac, but I do not always want to use TDM. Often I want to do something else on the iMac.

Just to clarify: is your iMac working in TDM running Catalina? Or just the mac using the display? AFAICT the first does not seem possible, so it is important to understand what is what.

Please elaborate.

There may have been a bug in a particular point release of High Sierra. I can't say.

FWIW, I would have been happy to test Target Display Mode with Catalina, unfortunately I don't have an iMac old enough to try that's nearby.