Nearly every Apple owner with an aging iMac has had the same thought while eyeing a shiny new Mac mini or MacBook Pro: why buy a separate monitor when I’ve already got a perfectly good 27-inch (or 24-inch) screen sitting on my desk? The answer is more complicated than a simple yes or no, and it hinges entirely on which iMac you own and when it was made.
Why This Question Is More Confusing Than It Should Be
Apple never built a universal “use as monitor” feature into iMacs the way many Windows all-in-ones or gaming monitors handle HDMI input. Instead, it shipped a narrow, temporary feature called Target Display Mode (TDM) that only existed for about five years.
- 2009–2013 iMacs: Full Target Display Mode support via Mini DisplayPort.
- Late 2013 to mid-2014 non-Retina iMacs: Still supported TDM via Thunderbolt.
- Late 2014 5K Retina iMac onward: TDM quietly disappeared and never came back.
- All Apple Silicon iMacs (M1 through the current M-series models in 2026): No TDM, no video-input hardware of any kind.
That gap matters because most people asking this question in 2026 own a Retina iMac from the last decade — meaning the feature they’re hoping for was killed off before they even bought the machine.
The Technical Reason Apple Killed Target Display Mode
Older iMacs had a physical video multiplexer chip that could switch the internal display panel’s input source between the iMac’s own GPU and an external Thunderbolt or Mini DisplayPort signal. That chip is exactly what let another Mac or PC “take over” the screen.
When Apple introduced the 5K Retina iMac in late 2014, the display required a custom timing controller (TCON) to drive its roughly 14.7 million pixels at high enough bandwidth. That TCON was built to accept signal exclusively from the iMac’s internal graphics processor — there was no longer a hardware path for outside video to reach the panel.
Apple’s own support documentation confirms Target Display Mode is only available on iMac models from 2009 through 2014, and that the feature does not exist on any Retina iMac.
The move to Apple Silicon in 2021 made this permanent rather than temporary. M-series chips are designed around a unified architecture where the display controller is tightly integrated with the SoC — there’s simply no video-input pathway built into the silicon, the ports, or the panel electronics of current 24-inch iMacs.
How to Actually Use an Older iMac in Target Display Mode
If you own one of the roughly 2009–2014 iMacs that does support this, the process is refreshingly simple — no software installation required.
- Confirm your iMac model supports TDM (27-inch models from Mid 2011 through Late 2013, and some late-2013/2014 non-Retina units).
- Connect a Mini DisplayPort or Thunderbolt cable from the source computer to the iMac’s Thunderbolt/Mini DisplayPort port.
- On the iMac, press Command + F2 (or Fn + Cmd + F2 on some keyboards) to switch into Target Display Mode.
- The iMac’s screen should immediately mirror the connected computer’s output at native resolution.
- Press Command + F2 again, or simply unplug the cable, to exit and return to normal iMac use.
A few caveats worth knowing:
- The iMac must be powered on and awake — it cannot act as a monitor from a sleep or shutdown state.
- You generally can’t chain Thunderbolt daisy-chained accessories through the iMac while it’s in TDM.
- Depending on the model, the iMac’s built-in camera, mic, and speakers may or may not be usable by the source machine — this varies by year and cable type.
- Using a modern USB-C source (like an M-series MacBook) usually requires a USB-C to Thunderbolt 2/Mini DisplayPort adapter, and results can be unreliable due to protocol mismatches.
What Actually Works in 2026 If Your iMac Doesn’t Support TDM
For the vast majority of people reading this — anyone with a 2015-or-later iMac — there is no native way to plug a cable in and turn the screen into a monitor. That said, several software-based workarounds exist, and they’ve matured considerably.
Software-Based Second Screen Tools
- Luna Display: A small hardware dongle (roughly $80) plugs into an HDMI or Mini DisplayPort port on your Mac, paired with an app on both machines. It essentially GPU-encodes the source Mac’s screen and streams it to the iMac over USB-C or Wi-Fi, turning the iMac into a wireless second display.
- Duet Display: A purely software app that does something similar without a dongle, though performance and latency depend heavily on your local network or cable quality.
- macOS Screen Sharing: Built into every Mac for free — you can remotely view and control another Mac’s desktop on your iMac’s screen, but this is a remote-control session, not true low-latency display extension, and it struggles with video playback or fast motion.
- AirPlay-to-Mac: Since macOS Monterey, any modern Mac can receive an AirPlay stream, letting you mirror an iPhone, iPad, or another Mac’s screen to your iMac wirelessly — again, this is streaming/mirroring rather than a native display extension.
The Key Tradeoff
None of these options give you the zero-latency, pixel-perfect signal that a real HDMI or DisplayPort cable provides. They all rely on compression and network/USB bandwidth, which introduces some lag — usually 20 to 80 milliseconds depending on the tool and connection quality.
- Good for: writing, spreadsheets, email, Slack, light browsing on a second screen.
- Not great for: fast-paced gaming, color-critical video editing, or anything requiring frame-perfect timing.
Weighing the Cost: Is Repurposing an Old iMac Worth It?
Before spending money on Luna Display or wrestling with screen-sharing settings, it’s worth comparing what you’re actually getting versus just buying a dedicated monitor.
| Option | Approximate Cost | Best For |
|---|---|---|
| Old iMac (2009–2014) + Thunderbolt cable | $0 (if already owned) | True native TDM, best quality, but limited to specific old hardware |
| Luna Display dongle + app | ~$80 one-time | Turning any modern Mac into a usable wireless second screen |
| New 27-inch external monitor (e.g., Dell, LG UltraFine) | $300–$700 | Permanent, reliable, native resolution with zero lag |
| Apple Studio Display | ~$1,599 | Mac-native color accuracy, but expensive relative to third-party options |
For anyone with a 2015 or newer iMac, buying a $300–$400 external monitor is almost always the more practical choice than fighting with software workarounds for years. The math changes a bit if you already own a pre-2015 iMac collecting dust — in that case, Target Display Mode genuinely does turn a “retired” computer into a legitimate, high-quality second screen at no extra cost.
What About Using a Current iMac’s Screen With an iPad or iPhone?
This is the reverse of the original question, but it comes up constantly in searches, so it’s worth addressing directly.
- Sidecar, Apple’s built-in feature, lets an iPad act as a second display for a Mac — but it does not work the other way around, and it has never supported turning an iMac into a receiving display.
- Universal Control, introduced in macOS Monterey, lets you move a single mouse and keyboard fluidly across multiple Macs and iPads sitting near each other — but again, this is about input sharing, not video signal sharing.
- Neither feature turns your iMac’s panel into a destination for another device’s video output.
People frequently conflate these features with Target Display Mode because they sound similar on the surface, but they solve a completely different problem: control sharing, not display sharing.
Conclusion
The honest answer in 2026 is that Apple made a firm design decision over a decade ago, and it hasn’t budged since: modern iMacs are sealed, output-only devices with no hardware path for accepting an external video signal. If you own a 27-inch iMac from 2009 through 2014, Target Display Mode still works exactly as it did back then, and it remains the single best way to repurpose an old iMac as a genuinely great secondary monitor. Everyone with a 2015-or-later model, including every Apple Silicon iMac on the market today, is stuck choosing between imperfect software workarounds like Luna Display or Screen Sharing, or simply accepting that a dedicated external monitor is the more reliable long-term investment.
FAQ
No. Apple Silicon iMacs have no video-input hardware whatsoever, so there is no cable, adapter, or macOS setting that lets another computer display its output on an M1, M2, M3, or M4 iMac’s screen.
It can, but only with the right adapter chain (USB-C to Thunderbolt 2 or Mini DisplayPort) and results vary by model — some combinations work smoothly while others produce no signal at all, so it’s more reliable to use two Macs from the same Thunderbolt generation.
It’s a solid solution for light productivity work like email, documents, and web browsing, but the compression-based streaming introduces noticeable lag for gaming, video editing, or fast-scrolling content, so it works best as a supplemental screen rather than your primary display.
