Every way to use an old iMac as a monitor.
Compared, with sources.

The quick decision

• iMac from 2009 to 2012: Encore (Late 2009 and up, 2007 to Early 2009 with the Legacy receiver) or Luna Display (from 2011, with a dongle). The newer paid apps start later.
• 27-inch 5K iMac, and you want native 5K: RetinaRelay or Luna Display claim it. Encore sends Retina iMacs at up to 2560 × 1440.
• You want a published latency figure: Luna Display and MacDisplay publish numbers from their own tests. Encore has not published one yet.
• Your main computer is a PC, or you also want an iPad: Luna Display or Duet.
• You want the iMac's keyboard and mouse working for your Mac over a cable, and its speakers too, for one small price: Encore is free during the public beta.
• iMac from 2019 or 2020, used now and then: AirPlay to Mac is built in.
• You never need the iMac as a computer again: a driver board turns a 27-inch 2014–2020 panel into a plain monitor.

The table

Prices and requirements as each maker listed them on 1 November 2026. "Oldest iMac" is the oldest model the maker's own requirements allow.

Apple's built-in options

Target Display Mode

The cleanest picture of all: a real monitor over a cable. But it only works with a Mac introduced in 2019 or earlier on macOS Catalina or older, per Apple's support page. The rules are in Target Display Mode explained.

AirPlay to Mac

Needs an iMac from 2019 or later on macOS 12 Monterey or later (TidBITS). It is built in and free. MacMost calls it "slower and less responsive" than a cable for everyday second-screen work. In practice that means the 2019 and 2020 iMacs.

Sidecar, Universal Control, Screen Sharing

Sidecar is for an iPad, not a Mac. Universal Control shares a keyboard and mouse between Macs but not a screen; Apple lists iMacs from 2017 and the 5K Late 2015 (Apple). Screen Sharing mirrors the other Mac instead of extending your desktop.

Apps

Encore

Encore is ours. It is free during the public beta: no account, no dongle, no subscription. It reaches the oldest iMacs: every Intel iMac from Late 2009 to 2020 on macOS 10.13 or later, and 2007 to Early 2009 with Encore Receiver Legacy. It works over Wi‑Fi, Gigabit Ethernet or Thunderbolt.

It is built from scratch: its own encrypted protocol, lossless tiles for still text, frames timed to the iMac's refresh, and Ultra with ProRes 4444 over a cable. Over a cable, the iMac's own keyboard and mouse can control your Mac, your Mac's sound can play on one or more iMacs at once, and when the iMac isn't a screen it shows StandBy: a clock with weather and your calendar.

Where Encore is weaker, plainly:

• No published latency yet. Our figures are design targets until we publish measurements per iMac model.
• Retina iMacs are encoded at up to 2560 × 1440, not native 5K.
• It needs macOS 14 or later on your Mac. RetinaRelay accepts 13, MacDisplay 12.3, Luna and Duet 10.13.
• macOS asks once before it opens the app. The install guide shows the clicks.
• It is young. The picture is still being checked model by model.

Luna Display

A small hardware dongle by Astropad that plugs into your main Mac. Astropad lists the USB-C unit at $89.99, supports iMacs from 2011, and needs macOS 10.13 on both Macs and a USB-C port on the main one (requirements). It does native 5K with the USB-C unit and works with PC hosts too. Astropad's support page states 1 to 4 ms wired and 7 to 25 ms on Wi-Fi; its 2018 test measured 11.3 ms over USB and 22.4 ms over Wi-Fi, on an iPad. Pick it for native 5K on a 2011-or-later iMac if you don't mind the dongle.

Duet

Subscriptions: on the US App Store, Starter $39.99 a year, Air $49.99 and Pro $79.99, or monthly. It has a page for exactly this job and also covers iPads, Android and PCs. Thunderbolt needs macOS 12.3 or later on both Macs.

RetinaRelay

Launched in October 2026: $49 once, one host Mac and unlimited iMacs, a 7-day trial. The host needs macOS 13 or later; the iMac must be Late 2012 or later on macOS 10.15 or later. It is built for cables and claims native 5K at 60 fps. MacGeneration covered it. Pick it for a 5K iMac on a cable at native resolution.

MacDisplay

€29.99 plus VAT, once, for three Macs, with a free tier limited to 10-minute sessions and 60 minutes a day. The receiving iMac needs macOS 11 or later, so 2014 or later. It is notarized, and its own photographed Ethernet tests show about 70–80 ms. It also shares one keyboard and mouse between the two Macs.

Two more

Yam Air Desk ($7.99 after a trial, receiving Mac on macOS 10.15 or later), and Spantop, which needs macOS 14 on Apple silicon and so doesn't run on an Intel iMac.

Hardware

A capture card (about $20) with an HDMI viewer app turns the iMac into a window onto any HDMI source. The iMac needs macOS 14 or later, so in practice a 2019 or 2020 model (9to5Mac).

A display driver board replaces the computer inside a 27-inch 2014–2020 iMac, so the panel becomes a plain 5K monitor. One kit we checked is listed at $449 on Macfixit's Australian store, and bare boards start around $170. The iMac stops being a computer: no macOS, no camera, no speakers. The full comparison.

Keeping the iMac's own system alive

OpenCore Legacy Patcher installs newer macOS versions on unsupported Macs, which can bring AirPlay to Mac to a 2012–2015 iMac. It works for many people, but it patches the operating system and has model-specific limits.

How we keep this fair

We list where others are better, link every number to its source and date the prices. If something here is wrong or out of date, tell us and we will fix it. Encore against each app, with where each one wins: Encore vs the alternatives, including a page per option such as Luna Display, Duet, RetinaRelay and Target Display Mode. To check your own model first, see the year-by-year guide, the compatibility page or the cable guide.