Pruning implies cutting back foliage, and this isn't usually necessary. What will be an ongoing job and absolutely essential (not to mention very tiresome) if your mint is planted in open ground is digging out the runners from beneath the soil, or the mint will take over the whole garden. Roots can go down 18 inches and runners can reach out 2 feet away and start growing there. Winter will prune it for you anyway - its deciduous, but if you like, cut it back with shears.

I grow my mint in a container....that way the roots are restricted and under control. I consider mint to be the most useful of herbs and so easy to cultivate. When the mint is looking a little tired, about August, cut back hard half of the pot and within a few weeks you will have a fresh supply. The other half will keep you going in the mean time. It is worth potting up some roots in another pot to keep over winter in the greenhouse....that way you usually have mint earlier than outside. Hope this helps.

Agree with Bamboo/ J58,I have two mints,one chocolate, both growing in 12 "pots .I cut them back hard 3 weeks ago as they were really leggy and now have nice ,new growth,best to keep them confined to pots as they can take over ,kerryvw.