Use a 2009 iMac as a second monitor.

Models

iMac (21.5-inch, Late 2009), iMac (27-inch, Late 2009)

Newest macOS

macOS 10.13 High Sierra

Runs Encore Receiver

Late 2009: yes. Early 2009: Encore Receiver Legacy (OS X 10.10 or later).

Cable

Gigabit Ethernet

Expected picture

Up to the panel's size, 30 to 60 fps (expected)

Target Display Mode

27-inch only, Mini DisplayPort (needs a Mac from 2019 or earlier)

AirPlay to Mac

No

What works

• An extra desktop: drag windows onto the iMac as onto any monitor.
• The iMac's own keyboard and mouse can control your Mac. Press Control-Option-Command-Esc on the iMac to switch back.
• Your Mac's sound can play on the iMac's speakers, on one or several iMacs at the same moment.
• More than one iMac at once, each with its own Connect button.
• When it isn't a screen, the iMac can show StandBy: a clock, the weather and your calendar.

Expected picture: up to the panel's size, 30 to 60 fps. We are still checking the picture model by model, so treat this as expected, not measured.

Which app goes on the iMac

The Late 2009 iMac runs up to macOS 10.13 High Sierra, so it uses Encore Receiver, which needs macOS 10.13 or later. The Early 2009 models stop at OS X 10.11 El Capitan and use Encore Receiver Legacy instead, which decodes in software and sends a smaller picture (about 1440 × 900). Your Mac needs macOS 14 Sonoma or later.

Which cable

Your iMac has a Gigabit Ethernet port and no Thunderbolt, so Ethernet is the way. You need nothing for Wi‑Fi. A cable is optional and helps most with full-screen video. Whatever you buy, the whole connection must handle at least 400 Mbit/s: Gigabit (10/100/1000) Ethernet or Thunderbolt, never a 10/100 adapter.

All cables and the traps to avoid

Target Display Mode on the 2009 iMac

Only the 27-inch Late 2009 iMac, over a Mini DisplayPort cable. The 21.5-inch Late 2009 is not on Apple's list. (Apple's list also names the 24-inch iMac from early 2009.)

Whatever the iMac, Apple requires the Mac that connects to be introduced in 2019 or earlier and to run macOS Catalina 10.15.8 or earlier. So a MacBook, Mac mini or iMac with Apple silicon can't use Target Display Mode. Source: Apple's support page.

AirPlay to Mac

No. Receiving AirPlay on a Mac needs an iMac from 2019 or later on macOS 12 Monterey or later.

Other ways

Paid apps, a capture card or a display driver board are other routes, each with its own iMac range. We compared them all, with sources, in every way to use an old iMac as a monitor. Why Apple's own option ended: Target Display Mode explained.

Questions about the 2009 iMac

Other years