How do I turn off monitor ?

Check System Preferences - Energy Saver and set the computer to sleep as NEVER.

 

Dumb me, I didn't know you could turn of just the LCD on an iMac but I guess it makes some sense, personally when not in use like when I'm out or at night I'd just put the whole computer to sleep unless the iMac needed to run 24/7.

 

You can't turn off the monitor of an iMac (just that). It has never been possible. Either sleep, or with the screen on. If you want too because you attach a serious external screen, the only thing to do is to just disconnect internal wires and hope it keeps working.

 

You can use the shortcut cmd+shift+eject to turn off the display. Use cmd+option+eject to put it in sleep.

 

In all honesty, I just have a Hot Corner set, top left, that when I place the mouse there, the display turns off. If you're using the computer and moving the mouse obviously when you place it there sometimes it will be an annoyance, so you can opt to have to press a Hot Key (option, command, etc) at the same time to create this action.

 

Before ML.. Ctrl - Shift - Eject is the shortcut to turn off iMac display without sleep or turn off the machine or distracting any operation. Try that and you'll see.

Now ML apparently changed the way some things work.

To the OP. Have you tried any other key combination? Cmd-Ctrl-Eject used to be for Restart and Cmd-Option-Eject used to be the one to Sleep your system. What do they do on ML?

 

You can also set it to use hot corners instead. easier in my opinion.

 

Hahaha I already said that! Idea stealer!

 

Sorry hahaha didn't even read your post. Great minds think alike.

 

I just tap the power button.