MacBook Pro and MacBook Air are two of the most popular Mac models Apple makes. Their easy portability means that you can use them anywhere — from the local coffee shop to a train and even the beach. Most of us, however, at least for some of the time, use them on a desk at home or in the office. And that often means hooking them up to an external display either to allow us to work from a larger screen and use the Mac in clamshell mode or to have a second monitor. Most of the time, it just works, but what do you do when your Mac is not recognizing an external monitor? We’ll answer that question here.

Why connect a MacBook to an external display

The main reasons for connecting a MacBook to an external display are:

  • So that you can work on a larger monitor with an external keyboard, mouse, and your MacBook closed.
  • So that you can use two displays.
  • To watch a movie playing on your Mac on a larger screen.
  • To use your Mac to deliver a presentation to a room full of people.
Tip

If you want to keep your Mac running smoothly in intensive workflows, running regular maintenance is essential. This includes clearing old junk files and clutter, updating all your apps, checking for malware threats, and optimizing performance. 

You can do all these things manually or use a tool like CleanMyMac — it makes the process simple and quick. Its Smart Care feature helps declutter your storage, neutralize malware, and speed up your Mac. It also checks for updates, so you don’t need to worry about using outdated software. Try CleanMyMac for free.




How to connect a MacBook to an external display

There are a number of different connections you can use to hook up your MacBook to an external display, depending on the ports on the MacBook. The most likely are:

  • HDMI
  • mini DisplayPort
  • USB-C
  • Thunderbolt
ports on the MacBook

HDMI is by far the most common, as most modern external displays and lots of recent MacBook Pros have HDMI connectors. If you have a MacBook Air, you can either connect it to the USB-C port on a display, if it has one, or use an adapter or dock to connect a USB-C or Thunderbolt port on your Mac to an HDMI or DisplayPort connector on a monitor.

Connecting by USB-C or Thunderbolt has the advantage that these ports can carry power and data as well as a video signal. So, you can use your display’s power supply to power your Mac and use its USB sockets to connect peripherals.

MacBook not recognizing external monitor: how to fix it

Normally, when you connect your MacBook to an external display and select the correct input on the display, you should see the Mac desktop on the display. You can then choose whether to use it as an additional display or to mirror the MacBook’s screen.

However, if your MacBook doesn’t detect the display, that won’t happen. There are a number of possible reasons your MacBook may not detect the display, and to fix the problem, we have to eliminate them one by one.

Tip

Not all cables are the same. Some USB-C cables, for example, won’t carry a video signal from your Mac to an external display. So, if you’re trying to connect to a monitor using USB-C and it’s not working, check that the cable you have is suitable. If it’s not, source one that is and try that. Similarly, if your MacBook is connected to a Dock by USB-C or Thunderbolt and the Dock is connected to a display by a DisplayPort cable, make sure the cable is an active DisplayPort cable, as some docks won’t work properly with passive cables.


To confirm that your Mac is not detecting the external display, go to the Apple menu > System Settings > Displays with the lid on your Mac open. 

System Preferences - macOS Displays settings

You should see two windows — one for each display. If you don’t, follow the steps below one by one.

  1. Unplug the cable from your Mac and plug it in again.
  2. Check that the correct input is selected in the display’s settings.
  3. Turn off the display and then turn it on again.
  4. Cut the power to the display and then power it on again.
  5. Restart your Mac.
  6. Try a different cable.
  7. If the display has multiple inputs, try a different one. For example, if you’ve been using USB-C, try HDMI.

If none of the steps above work, try connecting the display to a different computer if you can. If it works on that computer, then you know the problem is with your Mac. If it doesn’t, the display may be faulty.

Assuming the display works with another computer, the next step is to check that macOS is up to date on your Mac by going to System Settings > General > Software Update and update it if it’s not.

How to fix other common issues

Mac detecting two monitors as one

Depending on the issue you have, follow the steps below one by one.

  • One monitor has a black screen

In this case, as well as all the possible causes below, it could be that there is a fault with the cable connecting your Mac to the monitor, with the monitor’s power supply, or with the monitor itself. These are the first things you should check. To do that, connect the monitor to a different computer and see if it works.

Tip

To learn more about how many monitors can be connected to different MacBooks, check out this article from Apple.



  • Both monitors show the same image

If both monitors show the same image or one has a black screen and you have confirmed that the monitor and the cable connecting it to your Mac are both working, first, restart your MacBook. If it does not help, follow the steps below: 

  1. Check how the monitors are configured. If the displays are set up in a ‘mirror’ configuration, they will both show the same image. Go to System Settings > Displays and look at the dropdown menu at the top. It should say Main Display or Extended Display. If not, click it and choose one of those options.
  2. Check the connection. If the monitors are connected to a docking station or hub, make sure it’s compatible with your Mac. Not all docks support dual monitors when connected to a Mac. If your Mac is connected to the dock (if you’re using one) or a monitor using a USB-C cable, make sure the cable is certified for carrying a video signal.
  3. Use Detect Displays. If, after you’ve updated macOS, your Mac still can’t detect two monitors, you can try to force it to see them. Go to System Settings > Displays. Press and hold the Option key. You should see the Night Shift option replaced with Detect Displays. Click Detect Displays.

Thunderbolt display not turning on

Here are some fixes to try if your Thunderbolt Display is not working. Follow them one at a time:

  • Try turning the display off and then on again. Make sure to disconnect all cables, wait for at least 30 seconds, and then plug them back in before trying to turn the display on.
  • Check that the cable is working. If you can get your hands on another cable, use that to connect your Thunderbolt Display to your Mac. If the Display turns on, it’s likely the original cable is damaged and needs to be replaced.
  • Reset your MacBook's SMC (System Management Controller) and NVRAM (non-volatile random access memory). This is performed differently with different hardware. If you have a Mac with Apple silicon, simply shut down your Mac, wait for 30 seconds, and turn it on again. These steps are not official, but users on forums say that they help reset SMC. For an Intel-based Mac, jump to this article.

Using your MacBook with an external display not only gives you a much bigger screen to work on but can also allow you to use two or more screens at the same time. If you work from home regularly, it’s almost essential — no one wants to be slumped over a laptop screen all day. All of that makes it frustrating when your MacBook won’t recognize an external display. However, if that happens to you, there are lots of things you can do to try and fix it. Follow the steps above in order, and you should forget about Mac not detecting second monitor or Apple monitor not turning on.