late 2009 27" iMac as second monitor for 2019 iMac

https://www.walmart.com/ip/USBC-To-Mini-Displayport-Cable-6Ft-USB-Type-C-Thunderbolt-3-To-DP-Cable-4k/158695909?wmlspartner=wlpa&selectedSellerId=18988&&adid=22222222227377794577&wl0=&wl1=g&wl2=c&wl3=471278591274&wl4=pla-974448610236&wl5=9008130&wl6=&wl7=&wl8=&wl9=pla&wl10=125210027&wl11=online&wl12=158695909&veh=sem&gclid=Cj0KCQiAh4j-BRCsARIsAGeV12DeV7WVmXAaABzrM-6J0TnMQZb2UkBt-2SRKmh8JYDTREkC2-YrAt4aAmPJEALw_wcB

All you need is this, Its really simple on the 2009 27s. I used that with my 2020 27 and 2017 27 which have the same ports and my 2009 27

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

"You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display."

A 27" 2009 or 2010 iMac in Target Display Mode connected via Mini DisplayPort does appear as just a normal display to the other computer, so the "Mac from 2019 or earlier" requirement doesn't apply. Your 2011 iMac, though, uses Thunderbolt, and besides not working with a Mini DisplayPort cable, does not appear a a normal display to the other computer. The other computer has to recognize it and that capability is not in any computer newer than 2019.

Hi, I have a similar question, I hope you can help---I want to also use my trusty 27" iMac (2009) as a display for a new Mac mini. I have all the cables, but haven't connected them. Should I first set the 2009 iMac as something else (like some sort of display mode)? And then turn them both on? I can't see how I can boot up the Mini without a monitor, as I want to use the iMac as the monitor. Does that make sense? Thank you!!

If your "new Mac Mini" is a 2020, or one with the Apple silicon processor, you can't. These are the hardware and system requirements for the now-deprecated (and unsupported) Target Disk Mode:

VIDEO SOURCE COMPUTER (the one sending the video signal to an external monitor):

• A 2019 or older Intel Mac running macOS 10.15 Catalina or older.
• Apple Silicon Macs are not supported
• mac OS11 Big Sur is not supported.

TARGET COMPUTER (the one to be an external monitor):

• An iMac 27-inch (Late 2009 and Mid 2010)
• any size iMac from Mid 2011 to Mid 2014.
• The target computer can run no higher macOS than OS 10.13.X High Sierra,

Hi, my bad, it's a 2011 (iMac 27), and yes it is running High Sierra; and yes, the MacMini is a 2020....but wouldn't the iMac just function as a monitor? or does it just not work that way? ...I would connect the 2 with a Mophie USB-C cable with Mini DisplayPort connector. I'm a nerd, but more of a graphics nerd, not tech. Thank you for your help!!