Sometimes /mentre/ can even be a translation for /as/, when it is related to coincidence in time. A well known example (for Italians who like songsby Fabrizio De Andre`) Original English (for instance, as is sung by Joan Baez, traditional ballad) As I walked out over London bridge, one misty morning early ... Italian well known version: Mentre attraversavo London Bridge un giorno senza sole ...

mentre == when only in cases where there are two (or more) events that share a common time. This will be the context that could guide you.

I am curious where you got the information about ' mentre == when ', it is not correct. mentre == while quando == when