I'm mostly right handed, yes I know that sounds a bit odd to many people to say you're mostly right handed, but for certain things I'm left handed and I don't take too much notice of switching from doing one thing with one hand, then something left with the other - it's just natural to me.
As far as I remember as a child I was right handed, using my right hand for writing, drawing, sewing and so on, until when I was around 12 years old when I started getting problems holding a (fountain) pen in my right hand for long periods. I tried writing left handed - not a great success, it felt awkward so I only did it to give my right hand a break then I gave up on the experiment after a while.
Four or five years later when I was learning to type (on a typewriter) I realized I was using my left hand much more than others did, especially for the keys in the middle of the keyboard. When computers with mice came along I discovered it was so much easier and natural for me to mouse left handed - oh the look of horror on IT persons' faces at work when they have had to visit and find the mouse of the left side of the keyboard. "You haven't changed the mouse settings, have you?" like it's a crime (I don't switch the buttons because my middle finger is stronger and less prone to joint problems than my forefinger).
I can be clicking through screens and doing stuff with the mouse with my left hand and have my pen poised in my right hand to write notes, annotate paperwork and so on. I have several colleagues who now think nothing of using the mouse with their left hand all the time, when they write with the right, after they've seen me doing it - me being a bad influence or just being practical? I'm all for what is practical, whatever works for you and thus the placing of phone, computer, document stand etc on my desk at work - I just wait to see if I can get the layout right for me again once we've moved office (some time in the next two months) and have new/different desks.