Can a mid 2010 iMac be used as a display for a MacBook Air?

You cannot use your iMac (21.5-inch, Mid 2010) as a Target Display for any Mac. For the 2010 model year, only 27-inch iMacs had the Target Display Mode feature. The Mini DisplayPort on your iMac is for output only.

Apple also later added restrictions such that no current or recent Mac supports using ANY iMac as a Target Display.

See: Use your iMac as a display with target display mode - Apple Support

If the unspecified MacBook Air is 2019 or older and running 10.15 or earlier and with mini DisplayPort capabilities, likely yes.

Otherwise, no.

Use your iMac as a display with target display mode - Apple Support

That Apple doc is AFAICT incorrect about the identity of the iMac 2010 as 24”. That year offered 21.5” and 27”.

Now as to whether you really want to use a display capable of 1920 by 1080 pixels (FHD, Full HD) from your MacBook Air is your decision. That resolution is probably one of the least expensive TVs or monitors available these days.

MrHoffman wrote:

Use your iMac as a display with target display mode - Apple Support

That Apple doc is AFAICT incorrect about the identity of the iMac 2010 as 24”. That year offered 21.5” and 27”.

That Apple Support document does not claim that there is a 24" 2010 iMac. The heading of the relevant section says "24-inch and 27-inch iMac models introduced in 2009 and 2010."

Apple released 24-inch iMacs in 2009, so those are the ones to which the first part of the heading refers. Since the heading does not refer to 20-inch iMacs (released in 2009) or 21.5-inch iMacs (released in 2009 and 2010), one can infer that they do not have the feature.

I’ve never seen any indication that 21.5” iMacs released before 2011 supported TDM.

The 21.5” iMacs that do support it all have 1920x1080 (non-Retina) screens, and require old-style Thunderbolt input. If Apple had not added restrictions on the source computer, you’d still need something like $80+ in specialized hardware to make a Thunderbolt connection from a modern Mac to one of those iMacs.

Servant of Cats wrote:

MrHoffman wrote:

Use your iMac as a display with target display mode - Apple Support

That Apple doc is AFAICT incorrect about the identity of the iMac 2010 as 24”. That year offered 21.5” and 27”.

That Apple Support document does not claim that there is a 24" 2010 iMac. The heading of the relevant section says "24-inch and 27-inch iMac models introduced in 2009 and 2010."

Apple released 24-inch iMacs in 2009, so those are the ones to which the first part of the heading refers. Since the heading does not refer to 20-inch iMacs (released in 2009) or 21.5-inch iMacs (released in 2009 and 2010), one can infer that they do not have the feature.

I am well aware of what hardware was produced.

That document does list 24” and 27” models in 2009 and 2010.

Both of which exist.

But I don’t see the iMac 2010 21.5” listed in the document.

Which means — per what is written — the model we are discussing here is not supported.

But I suspect that literal reading is incorrect.

That your assumption is correct.

That somebody mis-identified or mis-worded the 24” reference.

Yeah, I’ve read a lot of support documents, various contracts, written a few too, and details matter.

That document does list 24” and 27” models in 2009 and 2010.

I do not think that is a typo. The previous versions of the TDM document had zero mention any 24-inch iMacs, just that only 27-inch 2009 and 2010 iMac were supported.

When the current version of the doc came out with "24-inch" I inquired behind the scenes to find out "what the heck?" I was told that the Early 2009 24-inch iMac A1225 was found to support TDM, so was correctly included.

I've never seen anything in the older TDM docs that I felt suggested the 21.5-inch model were supported before 2011, but maybe that's just me.

Allan Jones wrote:

That document does list 24” and 27” models in 2009 and 2010.

…

I've never seen anything in the older TDM docs that I felt suggested the 21.5-inch model were supported before 2011, but maybe that's just me.

Seems the original poster here gets to try it, and let us know. I love it when a plan comes together.