Board Thread:Questions and Answers/@comment-28314096-20160426201603/@comment-16461120-20160531233027

If I had to guess, I suspect it was like many things MARVEL where the "real name" is first revealed in a handbook, in this case has an entry for "Thor's Hammer", which begins with noting that it is in fact named Mjölnir. Whether or not it was ever called that in an actual comic during the two decades prior, I couldn't tell you.