Over the years, Marvel has explored a lot of characters and possibilities using alternate realities. Now, there are a whole lot of alternate universes in the Marvel Multiverse, and characters now frequently cross over between realities and timelines.
Excluding What If? and obvious parody comics, what is the earliest mention of an alternate reality in Marvel comics?
Possibly the Avengers meeting the Squadron Supreme in Avengers #85 (Feb. 1971).
One of the first ongoing series that explicitly took place in an alternate future would be Killraven, starting in Amazing Adventures vol. 2, #18 (May 1973).
The alternate dimension of Kalahia first appeared in 1940, in Mystic Comics #4. Mystic Comics was published by Timely Comics, which was later renamed Marvel Comics.
