Mionix Castor Review — Great Design, Subpar Software

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In Greek mythology, Castor was the twin of Pollux, and both brothers helped Jason reclaim the powerful Golden Fleece. As a gaming mouse, the Mionix Castor ($70) falls a little short of its mythological namesake.

Although Castor continues Mionix's reputation for crafting supremely comfortable mice, it also falls prey to software that feels behind the curve, just like its predecessors, the Avior and the Naos. The Castor is extremely comfortable and performs well in-game, but there are better options in its price range that provide more robust software components.


Mionix has a reputation for excellent mouse craftsmanship, and the Castor lives up to the company's lofty pedigree. The Castor is a small mouse with a relatively high profile, ideally suited to claw-grip players but just big enough to give palm grippers something to work with. There's a textured spot for the thumb, although the opposite side is completely smooth.

The Castor features an acceptable array of six buttons: a left button, a right button, a clickable scroll wheel, a dots-per-inch (DPI) sensitivity adjuster and two thumb buttons. Each one is programmable, although you'll probably want to change only the functions on the two thumb buttons, and each is in a convenient location and provides supple resistance.

Beyond that, the Castor is smooth and black all around, with illumination on both the scroll wheel and the palm rest. It's an attractive mouse that's pleasant to hold for hours on end.


In previous reviews of Mionix mice, I found the Avior and the Naos to have commendable ergonomic designs but software that's extremely easy to crash and comparatively difficult to fix. The Castor software gets higher marks than its predecessors, as it's more robust and much easier to navigate. However, it still could use some improvement.

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The Castor employs device-specific software that controls this mouse and nothing else. Arguably, it's not as versatile as mice like the Razer Synapse 2.0 or the SteelSeries Engine 3. But considering Mionix produces only one keyboard and one headset, a comprehensive ecosystem is probably not necessary.

The software is functional, if a little unremarkable. Typos like "costum color" make it feel a little sloppy, but it's still fairly easy to get around and customize your options. You can reprogram any of the buttons and choose from millions of different colors for the illumination. The backlighting is one of the more accurate schemes I've encountered on a mouse, nailing even elusive oranges and yellows. You can also optimize the mouse for your desktop or mouse pad, as well as adjust the DPI to between 100 and 10,000 — a very generous range.

The program's only major design flaw is that you cannot link the five possible programmable profiles with games or other programs. Instead, you'll have to manually select the appropriate profile before you launch a title. This is a minor annoyance, but considering how easy game-linking is when using Logitech and Razer mice, it feels like a key missing feature. 

During my initial tests, the Castor software was buggy to the point of being unusable. I tried three times to set up four profiles for our test games; each time, upon reaching the fourth profile, the program crashed and would continue to crash each time I tried to adjust anything.

I contacted Mionix about the issue, and learned that my problems arose because I exceeded a character limit when naming mouse profiles. Fortunately, Mionix has since patched the issue, and I haven't experienced any profile-related crashes since. It doesn't change the fact that the Castor's software isn't as robust as that of its competitors, but it's still the most reliable Mionix app we've used yet.

The Castor also lacks the type of physical customization offered by Logitech's $80 G502 Proteus Core, which gives you fine control over the mouse's heaviness via detachable weights. It's not a deal breaker, but it's something to consider if you're a gamer who wants to personalize your mouse as much as possible.


I tested the Castor with Titanfall, StarCraft II: Heart of the Swarm, Batman: Arkham Knight and Star Wars: The Old Republic. Once I got the profiles configured correctly, each game worked very well. The Castor is a very capable mouse across a variety of genres, and it worked especially well for titles that didn't need many extra buttons.

Titanfall and Heart of the Swarm, for example, were the best matches for the Castor's general design. In Titanfall, I used the extra buttons for my futuristic melee abilities, while in Heart of the Swarm, I ordered my Terran soldiers to move across the map, attacking enemies they encountered along the way.

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Like other mice with few buttons, the Castor worked just fine for The Old Republic at a casual level, but probably would not suit high-level players who need to access tons of skills within thumb's reach. Massively multiplayer online (MMO) diehards should probably look into a mouse with more buttons (and more robust software), such as the Razer Naga Epic Chroma ($99).

Bottom Line

From a design perspective, the Castor is Mionix's best mouse yet. The lighting is beautiful, the ergonomics are comfortable and the button layout makes a lot of sense. Its in-game performance is beyond reproach.

With an $70 price tag, however, the Castor is asking a lot. The same amount of money would buy you the almost perfect Logitech Proteus Core, as well as the excellent Razer DeathAdder or the SteelSeries Sensei. The Castor is still a dependable gaming mouse, but its aforementioned competitors have better features and software.

Marshall Honorof

Marshall Honorof is a senior editor for Tom's Guide, overseeing the site's coverage of gaming hardware and software. He comes from a science writing background, having studied paleomammalogy, biological anthropology, and the history of science and technology. After hours, you can find him practicing taekwondo or doing deep dives on classic sci-fi.