I’ve been holding a mouse on average for 12 hours a day for past 15 years and never had an issue. My experience pretty much confirms @sundialsvc4 observation.
Most of my colleagues who had some wrist issues were using their mice in a really dumb ways. From the data I gathered most of the people who had issues had their entire forearms resting on the table, and were moving the mouse with their wrist. On top of that, their mouse sensitivity was often ridiculously low, requiring them to twist their wrists way too far to actually move the cursor even half the screen. That puts a LOT of strain on your wrist.
Here’s what I am doing past 15+ years without any issues at all (many of the points repeat @sundialsvc4 points):
1, Make sure your chair armrest is the exact same height as your desk surface.
2, Invest in a good gamepad with no friction (I have razer destructor 2 and it’s fine) and most importantly, a really good, accurate gaming mouse. I have Zowie EC1-A (EC2-A these days). Why a good accurate gaming mouse? Because they you can use it at really high sensitivity but still be accurate, which brings me to:
3, Make sure your mouse sensitivity is high enough, and learn to use it. It will take about 3 days for your muscle memory to compensate, but if you have a good, high polling rate, high DPI gaming mouse, you can be extremely accurate even at high sensitivity. This allows you to do one important thing - you can control your mouse with a fingertip grip with your fingers mostly, with your wrist being almost completely static (I use something half way between palm and fingertip grip). I can get from the left side to the right side of my 1920x1200 screen with my wrist not tilting more than 5°
You can make a simple test to test your mouse sensitivity:
A, Put your mouse cursor to the very left edge of your screen
B, Move the cursor at a constant speed directly from the left to the right side of your screen so that it takes exactly 3 seconds to get there (constant, 3 second long travel minimizes the impact of OS mouse acceleration).
C, If your mouse moved more than 5cm, your sensitivity is too low.
But keep in mind that if you change sensitivity high enough with some cheap shitty 125Hz polling rate mouse, you’ll have no chance of using it precisely at that sensitivity. So it really relies on quality mouse.
4, The way I am holding my mouse is that my wrist rest fully on the wrist bone that’s below the pinky.
And this contact point is just one to two centimeters from the desk edge. That makes it really easy to just lift slightly and move the entire arm rather than twist your wrist. Good gaming mouse ensures that the wrist is slightly tilted so that the nerves coming through are not compressed. The elbow rests on the armrest in the same height as the desk.
Combination of these factors will pretty much allow you to minimize strain on your wrist, simply because you won’t be moving your wrist almost at all, and if you are, you won’t be doing it in a manner that compresses the carpal tunnel nerves.
I’ve seen people complaining that even vertical mouse did not help, just to notice their sensitivity on it was so low they had to drag it over the entire mouse pad to make it half way through the screen 
Also, just keep in mind that if you try the steps above, it won’t magically be better in an hour. The body and muscle memory will still need to adjust to a new positions, so it may not be as comfortable from the start.
Some other things that helped me: a gel pad to rest my hands on while typing and some gadgets (gyroscopic balls and those rubber rings that guitar players use) to strengthen the muscles in my hand - after it got better of course. Also don’t forget your other hand. Its not as bad as you think…
Out of curiosity, while mousing, are you left or right clicker? 
LSI to all the deviants
