Stan Lee’s first Marvel superhero co-creation was the Golden Age character the Destroyer (Keen Marlow), introduced in Mystic Comics #6 in 1941.

Who the character was

The Destroyer was a costumed American journalist operating behind enemy lines during World War II, empowered by a super-soldier-like serum given to him in a Nazi prison. He fought Nazi forces in occupied Europe and was published by Timely Comics, the company that would later become Marvel.

Why this counts as Stan Lee’s first

Stan Lee had earlier non-superhero work and supporting characters, but sources identify the Destroyer as his first actual superhero co-creation, done with artist Jack Binder. Later and more famous creations such as the Fantastic Four and Spider-Man came decades afterward in the 1960s.