Using iMac Mid 2010 as a display for my 2017 MacBook Pro

The 2010 iMac does not have Thunderbolt interface so using any

Thunderbolt cable will not work.

What you need is a 3rd party generic USB-C to Mini DisplayPort adapter.

https://www.amazon.com/s?k=usb-c+to+mini+displayport+adapter&crid=1CSHF72E42SN5&sprefix=usb-c+to+mini+disp%2Caps%2C140&ref=nb_sb_ss_i_1_18

You then need a MiniDisplay Port cable (not Thunderbolt).

The various knowledge base articles really need to be updated to

reflect this.

On the Thunderbolt3 to Thunderbolt2 adapter page it states:

What is confusing is that on the Apple site both Thunderbolt and Minidisplay should work:

The chart says a 2010 21.5-inch iMac does not support target display mode. Note that support for the smaller screen size did not start until the following year (third line in your screenshot). Originally TDM was unique to only the 27-inch iMacs, including 2010 products, so your will not work that way.

Allan Jones is 100% correct a 21.5" 2010 iMac will NOT work in Target Display Mode, if it was a 27" model it would work assuming you got the correct cabling which you did not.

Oops, missed that the OP had a 21.5 2010 iMac.

Probably didn't have enough coffee at the time.

So I am not the only one who surfs here when low on caffeine? What a relief! 🤪