Second Display is one app with two roles. Download it once and open it on both Macs.

On the Mac you use as a display

  1. Open Second Display and choose Use this Mac as a display. On an iMac this role is already selected.

  2. Plug a Thunderbolt cable between the two Macs, or choose Use Wi‑Fi or Ethernet instead.

  3. The Mac now says “Waiting for your Mac…”. Its window goes full screen when your Mac connects.

On the Mac you work on

  1. Open Second Display and choose This is the Mac I work on.

  2. Allow Screen Recording, and on macOS 15 or later Local Network access. Why Second Display asks

  3. Plug in the cable. A Thunderbolt cable shows “Thunderbolt cable detected”.

  4. The two Macs find each other on their own, and a speed test of about five seconds recommends the quality.

  5. Sign in with your email, with Apple or with Google. This starts the 7-day free trial; you’ve already seen the iMac work by then.

  6. Done: “Your iMac is now a display.” Find Second Display in the menu bar.

Second Display lives in the menu bar of the Mac you work on. If you turn off Show in menu bar in Settings › General, open the app again to reach its settings.