The late 2015 iMacs, with Retina 5K, Retina 4K or the standard 21.5‑inch screen, are the oldest iMacs that can still be a display for a newer Mac. Their ports are Thunderbolt 2, which look like the older Mini DisplayPort, not like the USB‑C ports on your MacBook. So a USB‑C cable doesn’t fit, and many people settle for Wi‑Fi. With one adapter and the right cable, the two Macs connect directly, and Second Display turns the iMac into a display for your MacBook.
Is this your iMac?
On the iMac, choose Apple menu › About This Mac. The model and year are shown under the name. In System Information, the late 2015 models are iMac16,1, iMac16,2 and iMac17,1.
| iMac | Ports | Second Display |
|---|---|---|
| 21.5‑inch, Retina 4K 21.5‑inch and Retina 5K 27‑inch, late 2015 | Thunderbolt 2 | Yes, with the adapter in this guide |
| Retina 5K, late 2014 and mid 2015 | Thunderbolt 2 | No: it can’t run macOS 12.3 |
| Other iMacs from 2013 and 2014 | Thunderbolt or Thunderbolt 2 | No: it can’t run macOS 12.3 |
| 2017 and later, and iMac Pro | Thunderbolt 3 (USB‑C) or later | Yes, with a Thunderbolt cable and no adapter |
The late 2015 iMacs run up to macOS Monterey, and the iMac needs macOS 12.3 or later, so install the latest Monterey update in System Preferences › Software Update. Apple: Identify your iMac model
What you need
- Apple’s Thunderbolt 3 (USB‑C) to Thunderbolt 2 Adapter. It works in both directions, so it connects a MacBook’s Thunderbolt port to an iMac’s Thunderbolt 2 port. Apple still sells it. Plug it straight into the MacBook: Apple notes that USB‑C hubs don’t pass Thunderbolt on. Apple: Thunderbolt 3 (USB‑C) to Thunderbolt 2 Adapter
- A Thunderbolt 2 cable, with the ⚡︎ symbol on both plugs. A Mini DisplayPort cable has the same shape but isn’t Thunderbolt, so it won’t connect the two Macs. Apple no longer makes its own Thunderbolt 2 cable; cables from other makers, new or used, do the same job.
- A MacBook with Thunderbolt and macOS 14 or later. Every MacBook Air and MacBook Pro with Apple silicon has Thunderbolt ports. A Mac with USB‑C ports but no Thunderbolt can’t use the adapter.
- Second Display on both Macs. The iMac needs no account and no Screen Recording permission. What you need
Connect them, step by step
On the iMac, open Second Display. Use this Mac as a display is already selected: continue, and the iMac waits for your MacBook.
Plug the adapter into a Thunderbolt port on your MacBook. Connect the Thunderbolt 2 cable from the adapter to one of the two Thunderbolt 2 ports on the back of the iMac, marked ⚡︎.
If your MacBook asks whether to allow the accessory to connect, click Allow.
On your MacBook, open Second Display and choose This is the Mac I work on. At How will you connect the Macs?, choose With a cable.
The two Macs find each other over the cable on their own, and a speed test of about five seconds recommends the quality.
Sign in to start the 7‑day free trial. Your iMac is now a display: drag a window onto it.
If the cable doesn’t connect, Second Display says what it sees and what to do. Thunderbolt didn’t connect: the cable messages
The picture over Thunderbolt 2
Over the cable you can choose any of the four quality levels: Low, Medium, High and Highest. Thunderbolt 2 carries less than the Thunderbolt ports of your MacBook, so the speed test measures your cable and recommends the level that suits it. You can pick another level at any time in the display’s Quality menu, and test again with Test Connection…. Quality levels and the speed test
For the sharpest text, leave the iMac at its own default resolution and choose Larger Text, Default or More Space in the display’s Resolution menu on your MacBook. Resolution and text size
No adapter? Wi‑Fi and Ethernet work too
Put both Macs on the same Wi‑Fi or Ethernet network and Second Display compresses the picture to suit the connection. The late 2015 iMac has an Ethernet port, which is steadier than Wi‑Fi. The picture isn’t encrypted, so use a network you trust, such as your home. You can start over Wi‑Fi today and add the adapter later: the cable takes over by itself when you plug it in. Use Wi‑Fi or Ethernet
Why not Target Display Mode?
Apple’s Target Display Mode works only on iMacs from 2009 to mid 2014, with macOS High Sierra or earlier, and only with a Mac from 2019 or earlier. A late 2015 iMac doesn’t have it, and a MacBook with Apple silicon can’t use it. Second Display does the same job on the iMacs that came after. Apple: Use iMac as a display with target display mode
More guides: an alternative to Target Display Mode, a 27‑inch iMac with Retina 5K as a monitor and an iMac as a dedicated monitor.
Questions
Can I plug a USB‑C cable straight into a late 2015 iMac?
No. Its Thunderbolt 2 ports have the smaller, Mini DisplayPort shape. Connect it with Apple’s Thunderbolt 3 (USB‑C) to Thunderbolt 2 Adapter and a Thunderbolt 2 cable, or use Wi‑Fi or Ethernet instead.
Does the iMac with Retina 5K from late 2015 have Target Display Mode?
No. Target Display Mode works only on iMacs from 2009 to mid 2014, and no iMac with Retina 5K has it. Second Display works on every late 2015 iMac.
Does Target Display Mode work with a MacBook with Apple silicon?
No. Apple’s Target Display Mode needs the other Mac to be from 2019 or earlier, with macOS Catalina 10.15.8 or earlier. Second Display works with a MacBook with Apple silicon on macOS 14 or later.
Will a USB‑C to Mini DisplayPort cable work instead?
No. That cable sends video out to a monitor, and a late 2015 iMac can’t take video in. Second Display needs the Thunderbolt connection that the adapter and a Thunderbolt 2 cable give you.
My iMac is from mid 2015 or earlier. Does it work?
No. The iMac with Retina 5K from late 2014 and mid 2015, and the iMacs from 2013 and 2014, can’t run macOS 12.3, which Second Display needs on the iMac.
Do I need the adapter for a 2017 or later iMac?
No. iMacs from 2017 on have Thunderbolt 3 ports, the same shape as your MacBook’s. Connect them directly with a Thunderbolt cable.
Can I use Wi‑Fi instead of a cable?
Yes. Put both Macs on the same Wi‑Fi or Ethernet network and Second Display compresses the picture to suit the connection. The late 2015 iMac also has an Ethernet port.