How to Use Your iMac as a Monitor for Another Device

Using an iMac as an external display is one of those features that sounds straightforward but comes with a surprisingly narrow set of requirements. Whether it works for you depends almost entirely on which iMac you own and what you're trying to connect to it.

What "Target Display Mode" Actually Is

Apple built a feature called Target Display Mode (TDM) into certain iMac models that allows the iMac to act as an external monitor for another Mac. When activated, the iMac's screen essentially becomes a high-quality display driven by a second machine — your iMac's own CPU and GPU step aside, and the screen shows whatever the connected computer is outputting.

This is genuinely useful if you have a MacBook that needs a larger display, or if you want to repurpose an older iMac rather than letting it collect dust.

The Hard Requirement: Model Year Matters Enormously

This is where most people run into a wall. Target Display Mode is not available on all iMacs. Apple officially supported it on specific models, and support was discontinued after a certain point.

If your iMac is from 2015 or later, Target Display Mode is not available. Apple removed it from 5K Retina models and never brought it back. There is no workaround, no firmware fix, and no third-party software that replicates it on unsupported hardware.

How to Activate Target Display Mode (Supported Models)

If you do have a compatible iMac, the process is relatively simple:

1. Connect the two Macs using the appropriate cable — Mini DisplayPort to Mini DisplayPort (for 2009–2010 models) or a Thunderbolt cable (for 2011–2014 models).
2. Power on both machines and make sure they're fully booted.
3. On the iMac you want to use as a display, press ⌘ + F2 (Command + F2). On some keyboards, you may need to hold the Fn key as well.
4. The iMac's screen should switch to show the output from the connected Mac.

To exit Target Display Mode, press ⌘ + F2 again, or disconnect the cable.

A Few Things That Can Prevent It From Working

• The iMac being used as a display must be logged in (it doesn't need to be at the desktop, but FileVault full-disk encryption before login can block TDM activation).
• Both machines need to be using compatible Thunderbolt or Mini DisplayPort ports — USB-C to Thunderbolt adapters can sometimes cause issues depending on the cable and adapter quality.
• The source machine (the one sending video) must support the same connector standard.

What About Connecting a Windows PC or Other Devices?

This is where things get more restrictive. Target Display Mode only works Mac-to-Mac. You cannot use an iMac as a display for a Windows PC, a gaming console, or any non-Mac device through TDM.

Some users have explored workarounds like screen-sharing software (such as Luna Display or other network-based solutions) to extend or mirror a display wirelessly from a PC to an iMac. These are fundamentally different from Target Display Mode — they work over Wi-Fi or USB and introduce latency, which makes them unsuitable for gaming or high-refresh-rate tasks but potentially acceptable for basic productivity work.

The 5K iMac Situation 🖥️

The reason Apple dropped Target Display Mode from the 5K iMac isn't arbitrary. The 5120 × 2880 resolution requires an enormous amount of bandwidth to drive — more than Thunderbolt 2 could carry at that time. Apple's own GPU handles the display through a proprietary, high-bandwidth internal connection that can't simply be handed off to an external machine.

Even as Thunderbolt 3 and 4 have become widespread, Apple has not reintroduced TDM on any modern iMac. Whether that's a technical decision or a product positioning one isn't publicly confirmed.

Variables That Determine What's Possible for You

Before assuming TDM will work for your setup, the key factors to assess are:

• Your iMac's exact model year and display size — the 27-inch 2014 model is different from the 21.5-inch 2014 model
• The macOS version on both machines — very old or very new OS versions can affect compatibility
• What device you're connecting from — Mac only, and it needs the right port
• Your use case — high-frame-rate gaming and video work have different tolerance for latency than basic document editing
• Whether a network-based software solution would meet your needs if your iMac isn't TDM-compatible

The gap between "my iMac could be a great second monitor" and "my specific iMac actually supports this" is wider than most people expect — and your model year is the single biggest factor in which side of that line you land on.