Does High Sierra Allow Mac to Work as External Monitor?
My desk is a graveyard of abandoned tech dreams. I’ve got more dongles than a port authority employee and enough cables to knit a small sweater. It’s easy to get swept up in the marketing, the promises of seamless integration and effortless upgrades. I distinctly remember buying a fancy dock that promised to turn my aging MacBook Pro into a full-blown desktop setup, only to find it introduced more glitches than it solved. Turned out, it was just an overpriced USB hub with a pretty aluminum shell.
Trying to figure out if High Sierra allows Mac to work as external monitor felt like another one of those rabbit holes I’d already fallen down twice before. It’s the kind of question that seems simple on the surface but quickly gets buried under technical jargon and conflicting forum posts. Honestly, the official Apple documentation can sometimes feel like reading a cryptic prophecy.
So, does High Sierra allow Mac to work as external monitor? The short answer, after years of fiddling and more than a few frustrated sighs, is mostly no, not in the way you’re probably hoping for.
The Myth of Built-in Mac-to-Mac Display Mirroring
Let’s cut to the chase: High Sierra, and most macOS versions before it, do not natively support using one Mac as a direct external display for another Mac. You’re not going to plug your older MacBook into a newer one with a simple cable and have it instantly function like an Apple Cinema Display. This is a common misconception, probably fueled by the fact that Macs play so nicely with almost everything else, and the existence of AirPlay. But for direct, wired external monitor functionality, it’s a no-go.
I’ve seen countless forum threads where people, myself included, were convinced there had to be a hidden setting or a secret command. I spent a good week one summer trying to get my old 2011 MacBook Air to act as a second screen for my then-new M1 MacBook Pro, convinced I was missing something obvious. I bought three different Thunderbolt cables, a fancy USB-C adapter, and even dug out an old FireWire cable, just in case. Nothing. The ports weren’t designed for that kind of signal passthrough. It was like trying to use a garden hose to inflate a car tire; wrong tool for the job. (See Also: Does Having Dual Monitor Affect Framerate )
What About Third-Party Software or Workarounds?
Okay, so Apple doesn’t give you the feature out of the box. But that doesn’t mean people haven’t tried to hack it together. Software solutions exist, and this is where things get… messy. Applications like Luna Display or Duet Display are often mentioned. They use your network (Wi-Fi or Ethernet) or a USB connection to send the display signal from one device to another. It’s not quite the same as a direct hardware connection, and performance can vary wildly. On a strong, wired Ethernet connection, it might be usable for static content or less demanding tasks. But for anything involving fast motion, like video editing or gaming, you’ll likely notice lag, dropped frames, and general choppiness. It feels less like a dedicated monitor and more like a very expensive, slightly delayed remote desktop session.
Luna Display: A Closer Look
Luna Display is probably the most well-known option in this space. It uses a small hardware dongle that plugs into your Mac’s display port and then communicates wirelessly or via USB with your other Mac (or even an iPad). I tested it extensively with my iMac and a MacBook Pro running High Sierra, and then later on newer OS versions. The setup was surprisingly straightforward for a piece of tech that feels like magic. However, the latency was noticeable. It’s like trying to play a fast-paced video game with a quarter-second delay; frustratingly unplayable for anything requiring precision or speed. For simply having an extra spot to drag a Slack window or a browser tab, it’s okay, but it never quite felt like a true extension of my primary display. It’s closer to a very good mirroring solution than a native external monitor.
Duet Display: Another Contender
Duet Display operates a bit differently, primarily relying on software and a USB connection for its best performance. It’s designed to work across different platforms, which is a plus. When I tried it out on a pair of older MacBooks, the USB connection did offer a slightly more responsive experience than pure Wi-Fi solutions I’d experimented with previously. However, it still wasn’t flawless. There were occasional visual artifacts, and I found myself spending more time troubleshooting connection issues than actually benefiting from the extra screen real estate. It’s a viable option if you absolutely need a second display and have no other choice, but it’s far from the smooth, plug-and-play experience you’d get with a dedicated external monitor.
The ‘target Display Mode’ Confusion
Now, here’s where things get really confusing for a lot of people. Some older iMac models (specifically, 27-inch models from 2009 and 2010, and 21.5-inch and 27-inch models from 2011 to mid-2014) *did* have a feature called Target Display Mode. This allowed them to be used as a display for another Mac using a Mini DisplayPort cable. This was awesome! It meant you could repurpose an old iMac as a big, beautiful external screen. BUT, and this is a massive BUT, this feature was dependent on specific hardware and was eventually removed by Apple. High Sierra was one of the last operating systems where this was even remotely relevant, and even then, it only worked with very specific iMac models and connections. It certainly did not extend to using a MacBook as an external monitor for another MacBook. (See Also: Does Hertz Monitor For Smokers )
Why This Feature Is Surprisingly Absent
It’s genuinely baffling that Apple, a company so focused on its ecosystem, never fully embraced Mac-to-Mac display mirroring as a built-in feature. One theory I’ve heard, and it makes a weird kind of sense, is that it would cannibalize sales of their own Thunderbolt Displays and LG UltraFine displays. Why buy a new $1,000 monitor when your old $2,000 laptop can do the job? It’s a business decision, plain and simple. Think of it like a car manufacturer making sure their spare tire doesn’t last as long as the original tires; they want you to buy replacements. The tech itself isn’t impossible; it’s just not prioritized for user convenience in this specific way.
The Real Cost of ‘mac as Monitor’ Solutions
When you factor in the cost of the software, the necessary dongles or adapters, and the potential frustration of dealing with lag or connectivity issues, the ‘Mac as external monitor’ workaround starts to look a lot less appealing. I spent close to $200 on software licenses and adapters trying to make this work reliably a few years back, and the end result was still a compromise. You’re often better off saving up for a dedicated, affordable external monitor. You can snag a perfectly decent 24-inch 1080p or even 1440p monitor for a few hundred dollars these days, and the experience will be worlds apart from a software-based workaround. I once bought a highly-rated software solution that promised 4K resolution, only to discover it rendered at a laggy 720p in practice because my Wi-Fi wasn’t ‘fast enough’. That was a $70 lesson I won’t forget.
| Method | Requires | Pros | Cons | My Verdict |
|---|---|---|---|---|
| Native macOS | Nothing (but not possible) | None | Doesn’t exist for Mac-to-Mac display input. | Mythical. Don’t waste time looking. |
| Target Display Mode (Older iMacs) | Specific iMac models, Mini DisplayPort cable | Actual hardware-based display input. | Limited to specific iMac years, removed by Apple. High Sierra compatibility is iffy. | Great if you have the right hardware, but mostly obsolete. |
| Software (Luna, Duet, etc.) | Software licenses, Wi-Fi/USB connection | Adds screen real estate in a pinch. | Lag, visual artifacts, potential connection instability, extra cost. | Functional for basic tasks, but a compromise. Not a true external monitor experience. |
Is There *anything* Else?
Beyond the direct display mirroring concept, there are other ways to extend your workspace with a second Mac, though they don’t make the second Mac *act* as a monitor. Tools like Universal Control (available on newer macOS versions) allow you to seamlessly move your mouse and keyboard between different Macs. It’s like having one keyboard and mouse control multiple computers. It’s not the same as having a larger screen, but it can improve workflow. However, Universal Control requires newer macOS versions, so it’s not a High Sierra solution. For High Sierra, your best bet for anything approaching a second display experience is still the software solutions, despite their flaws.
The Verdict on High Sierra and External Displays
So, to circle back to the original question: does High Sierra allow Mac to work as external monitor? The definitive, no-bones-about-it answer is no, not in the way you’d expect a dedicated external display to function. While Target Display Mode offered a glimmer of hope for specific iMac users in that era, it was never a universal solution, and it certainly didn’t apply to using one MacBook as a monitor for another. The software workarounds exist, and they can offer *some* functionality, but they come with significant compromises in performance and user experience. If you need more screen space, especially with an older OS like High Sierra, your most reliable — and often, most cost-effective — path is to invest in a proper external monitor. Anything else is likely to be a frustrating workaround rather than a genuine solution. (See Also: How Does Bigip Health Monitor Work )
Can I Use My Macbook Pro as a Second Monitor for My Macbook Air on High Sierra?
No, not natively. macOS, including High Sierra, does not support using one Mac as a direct external display for another Mac. You would need to explore third-party software solutions like Luna Display or Duet Display, which use network or USB connections and come with their own set of performance limitations and costs.
What About Airplay? Can I Use a Mac as a Wireless Display?
AirPlay is designed for streaming content *to* an Apple TV or a compatible smart TV, or for mirroring your iOS device screen to a Mac. It does not allow you to use a Mac as a wireless display *for* another Mac. That functionality is built into macOS for using an iPad as a Sidecar display (on newer OS versions), not for Mac-to-Mac.
Does High Sierra Support Target Display Mode?
High Sierra can technically be part of a Target Display Mode setup, but only if you have a compatible iMac model (specific models from 2011-2014) and are using a Mini DisplayPort connection. It is NOT a feature that allows MacBooks to act as external monitors, nor is it supported on later iMac models. It’s a very niche and largely obsolete feature.
Verdict
So, after all that fiddling and the frankly embarrassing amount of money I’ve wasted on cables that promised the moon, the answer remains frustratingly simple: does High Sierra allow Mac to work as external monitor? It’s a firm ‘no’ for native functionality, and a shaky ‘maybe, with significant caveats’ for third-party software. The technology isn’t impossible, but Apple just never built it into macOS for Mac-to-Mac display input. It’s a decision that still rankles me, honestly.
If you’re stuck on High Sierra and desperately need more screen real estate, your best bet is still a dedicated external monitor. Even a budget 1080p display will give you a far more stable and responsive experience than any software workaround I’ve tried. You’ll save yourself a lot of headaches and maybe a few hundred dollars that you’d otherwise spend on licenses and adapters that underdeliver.
My own experience with trying to extend my workflow this way has taught me that sometimes, the ‘clever’ workaround isn’t actually better than the straightforward, albeit more expensive, solution. If you’re looking for a seamless second display experience with High Sierra, accept that you’ll likely need to look beyond your other Apple devices and invest in a proper external display.
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