EWTN Catholic Q&A
Name Pope Takes
Question from Richard on 04-19-2005:

Why do Cardinals change their names when they are elected Pope?

Answer by Colin B. Donovan, STL on 04-26-2005:

It started with Pope John II (533-35), whose given name was Mercury (he was a convert). As it was not a fit name for a Pope he took John. Subsequent popes have followed his example of changing their name.