I think mint does better outdoors.  Yellowing can be overcome by adding a fertilizer with iron chelate in it.  And yes, overwatering is no good.  Mint is a cold  climate herb so it is not surprizing that it does well in Canada.  It thrives in relatively poor soil and in shade.

I have a spearmint plant in a clay pot.  About a month and a half after moving it to the pot from the little plastic one I bought it in, the leaves are starting to turn yellow around the edges.

Any ideas what could be going on?  It's kept outdoors, in a spot that gets sun for part of the day, but not really direct sun, since it's surrounded by buildings.  I've been watering it when the soil feels dry, if it doesn't rain, and it did rain a lot in the last month and a half, stopping only in the last couple of weeks.