Board Thread:Questions and Answers/@comment-28314096-20160426201603/@comment-61022-20160601061920

That's a tough call... That's one of those things where it's one of those long standing things, where it's been called Mjolnir and yet you go back far enough they'd call it simply "Thor's Hammer" or "Thor's Enchanted Mallet" to pinpoint when they specifically referred to it as Mjolnir is hard to peg down.

It was still during the Lee/Kirby era. Shortly after the Journey into Mystery was retitled to Thor. Somewhere around or so.