Mighty Mouse wasn't always using that name. Initially, Paul Terry had christened him Super Mouse, as he was always meant to be a parody of Superman. However, in 1944, after 2 years of shorts, Terry learned that Timely Comics (now Marvel) already had a copyrighted character named Super Mouse, forcing the name change. Subsequent reissues of the 1942-4 shorts included new title cards and dubbed over narration.
Here, then, is the short that started it all. "The Mouse of Tomorrow". Castle Films got their hands on a negative of the original, and it is a collector's dream. The narrator, it appears, would be radio (and later TV) announcer Ken Roberts:
Primitive, yes, but you have to start somewhere, right?