Several 'prototype' characters were introduced in stories in this title - that is, characters with themes or origins which would be re-used or echoed by later Marvel superheroes and villains. For example, #73 featured a story called "The Spider" which features a spider bombarded with radiation and becomes partly human; this is of course very similar to the origin of Marvel's flagship character Spider-Man. Three issues earlier, in #70, a character appeared who very closely resembled Spider-Man's later enemy, the Sandman. And there were others.
By 1962, Marvel was starting to change their direction away from straight fantasy and more toward superhero characters with continuity; this was facilitated mainly by the success of Marvel's first successful Silver Age superhero title, Fantastic Four, which debuted in an issue cover-dated November 1961. Before long the company was introducing more and more such characters within their various titles.
Issue #83 of Journey Into Mystery, therefore, was chosen to launch a new type of superhero character, Thor the Asgardian god of thunder. Despite the character's mythical origins, writer/editor Stan Lee and artist Jack Kirby fit Thor into the more generic superhero mold then being worked out within the company's titles: for his debut adventure, for example, Thor battled against an army of rocklike alien invaders called the Stone Men from Saturn.
As time progressed, the mythology (pun intended) of Thor and his fictional universe was worked out by the writer-artist team. The strip gained a plethora of characters, even an entire fictional world, Asgard, the home of the gods, against which the characters could operate. In #97, a backup strip behind the main adventure called Tales Of Asgard began, which instructed readers as to the (fictional) history of this made-up world.
After 125 regular issues and one King-Sized Annual, Journey Into Mystery morphed to become the title of Thor, irreversibly devoted to the Thunder God's adventures. For a brief time between 1996 and 1998, the regular Thor title was reverted to Journey Into Mystery, but even that ended with issue #521 in June - after which an all-new Thor title was launched starting again at issue #1.
A second series called Journey Into Mystery went through 19 issues in the early 1970's; for the first 5 issues it featured original horror/fantasy stories as well as some adaptations of the works of authors such as Robert Bloch and H. P. Lovecraft; starting with #6, however, the book consisted of reprints from the original series.