Can I use an external display while turning off an iMac’s built-in screen?

Yes, where connection management is supported, BetterDisplay can soft-disconnect the iMac's built-in screen after macOS is running. This removes it from the desktop layout while the app is managing it. You still need a working way to reach the Mac's startup and login screens.

First confirm that you can log in and use the external display. In its Manage Display menu, choose Set as Main Display. Then open Settings > Displays > Overview > Display Connections > Connection Management Settings… and review Allow disconnecting the built-in display of the iMac. This Pro option is off by default because an iMac has no lid or built-in display cable to reconnect if recovery is needed.

If you enable that option, use the built-in display's header connection switch to disconnect it, or use an explicit Connect or Disconnect a Display action. Check the header switch's configured behavior if it performs a different operation. Keep the external display connected and usable.

A login automation can request the disconnection again after login, but cannot make it apply before BetterDisplay can run. If the internal panel has failed and the external screen does not show the boot or login environment, this workflow alone cannot restore access. Establish a usable login or already-configured remote-access path before relying on the setup.

See Display Connections for safeguards and recovery limitations. Soft disconnection is not a repair for a failing panel or a Mac that produces no image during startup.