How to connect an iMac 27inch from mid-2010 to a MacBook Pro 2023 ?

I have HDMI

Well, technically maybe, that might be an option - but this requires the Kanex XD video converter (if you can even find one now) which only supports the HDMI mode at 720 on that model iMac which is only half the resolution that the 27" iMac is capable of at 1440. So, it looks really crappy.

I had that same model 2010 iMac and used it to connect to an XBOX through the Kanex XD, it really did not look very good to dumb down the display to half of the native resolution. Very disappointing.

https://www.kanex.com/xd

Just posting this for historical reference, sounds like you have arrived at the best option already.

Apparently…

iMac (27-inch, Mid 2010) - Technical Specifications - Apple Support

To add, while this should work for you, from a certain perspective the use of an old iMac as a monitor is not an ideal situation. Consider that a fully capable, vintage all-in-one computer will eat much more energy than a dedicate contemporary monitor. And should the iMac startup drive fail - a real possibility at any time due to age - then the ability of the iMac to act as a monitor may be compromised. In fact, should any component of the fifteen (!) year old iMac fail, then it may no longer work as you hope, and repairs will be unlikely.

Still, if you're willing to overlook these things, then this may fit the bill for a while - if it works.

dialabrain wrote:

Isn't the Mini DisplayPort on the iMac output only?

In the spec sheet it's noted that input from DP sources is supported, so I suppose not.

And I've read at least a couple posts of users who have successfully leveraged the feature.

zebumax wrote:

Thanks a lot to all.

I suppose that buying a specific screen will be the best solution.

Speaking for all, you're welcome, my friend.

And yes, all things considered, buying a dedicated monitor is probably the wisest choice. 👍🏽

Though, should you decide to give a shot to the USB-C to DisplayPort option, please do come back and let us know the outcome. 🙂

Re: “I suppose that buying a specific screen will be the best solution.”

Back when the first 27” iMac came out in Late 2008, 27” screens were not common. A 27” 2560x1440 Dell monitor sold for $1200 - or $1000 when on sale. Adjusted for inflation, that’s about $1500 to $1800 in 2025 dollars; an amount that now might buy you a 27” 5K Apple Studio Display, or three to five decent 27” 2.5K or UHD 4K monitors.

The wild card here is Apple. The 2010 iMac is an obsolete device per Apple's classification system. There is nothing to prevent Apple changing operating system code to block this USB-C connectivity exploit to old devices in a current or future release of macOS.

We have all seen Apple remove useful features from the operating system and hardware over the years. I don't think they are done with that quest…

If any are interested, this link is to a Community discussion earlier this year on the subject. The author was using a 2009 27" iMac as monitor for a 2024 Mac mini M4. Is there a switch or toggle for a Mini-Di… - Apple Community

Per the specs for that iMac - iMac (27-inch, Late 2009) - Technical Specifications - Apple Support - it could ❝support input from external DisplayPort sources (adapters sold separately).❞

Isn't the Mini DisplayPort on the iMac output only?

So have you personally seen this setup work? Otherwise I think this unhelpful to a user asking for advice.

Could, maybe, might possibly...