In a nutshell, here’s how to run a MacBook with just an external display:
- With your MacBook on and running, plug in your external monitor and turn it on
- Make sure the MacBook display is in mirror mode.
- Close your MacBook. The whole system (including external display will go to sleep).
- Plug a USB device into the MacBook (e.g. your mouse) to bring it out of sleep
- Open up your MacBook. The display will be off.