Set the Primary Display on a Dual-Screen Mac Setup

If you are running a dual-display setup, you can easily adjust the primary display monitor in Mac OS X. When would you want to do this? For example, if you have a MacBook Pro 13″ hooked up to a larger external display, and you want the external display with it’s higher resolution to become the primary display, and your MacBook Pro with it’s smaller resolution to become the secondary display. This is just a matter of settings adjustments and it only takes a minute to configure, though it’s not particularly obvious at first glance.

Let’s walk through how to set the primary screens on a multi-display Mac setup.

How to Set the Primary Display on a Mac

You will obviously need an external display to use this feature. Before beginning, have both displays turned on, and the external display already connected to the Mac:

1. Open System Preferences from the Apple menu 
2. Click on the Display icon
3. Choose the ‘Arrangement’ tab
4. Click and hold on the white bar at the top of the current primary display, this white bar signifies the menu bar
5. Drag the white bar across to the other monitor you want to set as the new primary display for your Mac
6. Notice the red border that forms around the new primary screen as you drag the white bar into the arrangement, this helps you determine which screen to use as the primary screen
7. After you release the white bar onto the other blue screen representation, the screens of both displays will flicker on and off briefly and the video output will adjust to accomodate the new settings
8. When satisfied with the new primary display setting, close out of System Preferences to keep the preferences set

This screenshot demonstrates the white bar actively being dragged from the built-in screen on the left to an external connected display on the right, notice the red border that indicates the secondary screen (right side) will become the new primary display.

Whatever monitor is set as the primary display will become the default display for applications launching, in addition to holding the system menubar, containing all the default desktop icons, and contain the Dock. Keep this in mind when adjusting screen priority, and do note that if the secondary display has become detached, the primary screen will revert back to a built-in display on any portable Mac model.

Using this method you can effectively turn any external display into the main display for any Mac (MacBook, MacBook Pro, Air, iMac, whatever) which is a really nice way to maximize screen real estate in dual-display setups that feature a smaller screened Mac with a larger external monitor hooked up to it. Note this is different than operating a MacBook or MacBook Pro in clamshell mode, which also will set the external display as the primary screen, though clamshell is different in that it disables the builtin screen of a laptop in order to power the external monitor.

You can do this with any external display, whether it’s an official LCD or LED monitor, an HDTV connected to a Mac through HDMI, or even something like a projector or a software-based solution like AirDisplay. If it’s recognized as an external display, it will work.

This is the same in all versions of MacOS and Mac OS X, be it MacOS Big Sur, MacOS Catalina, MacOS Mojave, Mac OS High Sierra, Sierra, Mavericks, Yosemite, El Capitan, or whatever else is running on the Mac.