How can I use iMAC 21.5" Mid 2010 as a mirror screen for a new MacBook Air 13" M3?

You can not use the Mid 2010 21.5" iMac as a display for a new MacBook Air in Target Display Mode.

You need to get an Apple or after-market external display to use with the MacBook Air.

see > Use an external display with your MacBook Air - Apple Support

Silviadrs wrote:

I am about to buy a new MacBook (Air 13" M3) and want to use my old iMAC (Mid-2010/21.5"/3.06 GHz i3/4 GB RAM/500 GB HD/256 VRAM/running HighSierra 10.13.6) to mirror the MacBook.

Ideally, I would love to have some sort of (economical) docking station where I can just plug in the MacBook and use my old iMac as the mirror (or a 2ºMonitor) and work with the iMac's keypad & mouse (so it's as if I have a new iMAC... I hate working on a small 13" screen).

I have checked, and my iMac does not seem to have Target Display Mode, but it does have a Mini Display Port. I would greatly appreciate any help or guidance.

You can't. Even if the old machine supports target display mode, your new one doesn't.

There are some software based solutions, but they tend to be less than ideal due to the lag involved. Just buy yourself an external monitor.

Your iMac (21.5-inch, Mid 2010) has a Mini DisplayPort output port, for connecting a second screen which has a resolution of up to 2560x1600 pixels to the iMac.

There's no support for using that iMac's screen as a display for another Mac, new or old.

Buy a real display.

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