How to use a MacBook as a second monitor for another Mac

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For a desk you use every day, connect the Macs over Thunderbolt or Ethernet. A wired connection carries the display stream over the cable, reducing your dependence on Wi-Fi conditions. MacDisplay gives you an extended desktop without a dedicated hardware device, using the compatible Macs you already own.

Give your old MacBook a job as a second display

An extra screen is useful when you need to see two things at once: write a report while keeping your sources visible, work through a spreadsheet with instructions open beside it, or keep a browser window next to your editor instead of switching back and forth. With an extended desktop, the apps keep running on your main Mac and the second MacBook provides additional screen space for their windows. That differs from mirroring, which shows the same content on both displays; Apple explains the distinction in its guide to extending or mirroring your Mac desktop.

MacDisplay creates an extended desktop between two Macs. Your main Mac needs macOS 12.3 or later, and the MacBook you use as the second display needs macOS 11 or later. Check both operating systems before choosing a cable: the receiving MacBook can run an earlier version of macOS than the main Mac, as long as it meets that requirement.

Can you use a MacBook as a monitor with HDMI?

An HDMI port on a compatible MacBook sends video to an external monitor or TV. It doesn't turn the laptop's built-in screen into an HDMI input for another computer, and Apple describes those video output capabilities in its guide to connecting an external display to MacBook Pro. To use another MacBook as a monitor, you need software that receives the image and displays it on that laptop: MacDisplay handles the connection and the desktop extension.

A Thunderbolt cable can connect the two Macs, but the app is still part of the setup. Also check the cable itself: a USB-C connector doesn't automatically mean it's a Thunderbolt cable.

Why we recommend Thunderbolt or Ethernet for everyday work

MacDisplay supports Thunderbolt, Ethernet, and Wi-Fi, and for a workspace you rely on throughout the day we recommend a wired connection. Wireless performance can change with distance, physical obstacles, and interference; sending the display stream over a cable removes those radio conditions from that connection's path. That's a practical advantage when the second screen is part of your regular workflow.

Your experience still depends on the computers, cables, and configuration: a wired connection reduces wireless variables, but it doesn't guarantee that every setup will perform identically. With MacDisplay, you can build your desk around Thunderbolt or Ethernet and use Wi-Fi when a cable is less convenient, choosing the connection that fits how you work.

Sidecar uses an iPad, not another MacBook

If you want to use a compatible iPad as your second display, Apple's built-in Sidecar feature is the right place to start: it extends or mirrors your Mac desktop onto the tablet, as described in Apple's official Sidecar guide. Sidecar doesn't use another MacBook as the receiving display. MacDisplay connects Macs to other Macs and currently doesn't extend a Mac desktop to an iPad, so if the device you want to reuse is a spare MacBook, MacDisplay lets you put that laptop's screen to work.

Universal Control doesn't extend your desktop

Universal Control lets you operate compatible devices with one keyboard and mouse or trackpad, but each computer continues to show its own desktop and run its own apps, as Apple explains in the Universal Control documentation. If you want to drag a window from your main Mac onto the other MacBook's screen, you need a desktop extension, and that's what MacDisplay provides.

MacDisplay also includes keyboard and mouse control for the other Mac, so you can use it to extend your desktop or to work across two separate computers with shared controls. Our guide on how to control two Macs with one keyboard and mouse covers that second workflow.

AirPlay and MacDisplay: a dedicated setup for working across two Macs

AirPlay can send content to a Mac configured as an AirPlay receiver. Availability depends on the devices, operating systems, and receiver settings, and Apple lists the details in its AirPlay to Mac requirements. MacDisplay brings several parts of a two-Mac workspace together: an extended desktop, Thunderbolt and Ethernet connections, a Wi-Fi option, file transfers, and keyboard and mouse control of the other computer.

For a second screen you use every day, we recommend MacDisplay over a wired connection: you can organize your workspace around Thunderbolt or Ethernet and reduce your dependence on wireless conditions for the display stream. You also have a way to send documents to the other computer within the same app, and our guide on how to transfer files between two Macs directly over a cable explains that part of the workflow.

How to use a MacBook as a second monitor with MacDisplay

Open MacDisplay on both Macs and decide which computer will be your main Mac: that's where your apps will run and where the extended desktop starts. On the MacBook that will act as the screen, choose “Become a monitor”; on the main Mac, choose “Extend screen”. Then follow the in-app guide to finish connecting and grant the required permissions. For a permanent desk setup, use a Thunderbolt or Ethernet connection suitable for your computers; Wi-Fi is also available when you prefer to work without a cable.

Once the desktop is extended, move a window you actually need onto the second screen: your reference document beside your writing app, your browser beside your editor, or your spreadsheet beside the report you're preparing. If your spare computer is a desktop rather than a laptop, read our guide on how to use an old iMac as a second monitor.

Try it with your own Macs before buying

MacDisplay has a free version with time limits, so you can test the connection before purchasing a license. Check text readability, move windows between screens, and use the apps you plan to keep on the second display. If you're building a wired workspace, test with the connection you intend to use, because an ordinary work session will tell you more about the setup than a demonstration on different hardware. The license costs €29.99 + VAT as a one-time purchase, with no subscription.

Frequently asked questions

Team MacDisplay MacDisplay Published on 12 October 2026

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