Rosen also moonlighted for other companies during this time: he was the (uncredited) letterer for the 1965-66 Archie Comics series The Mighty Crusaders.

Sam Rosen began his career as a comic book letterer in New York City in 1940, working on writer-artist Will Eisner's The Spirit, the seven-page lead feature in a comic-book style Sunday supplement syndicated to newspapers.

Beginning with his first confirmed Marvel Comics credit, the three-page recap "The Origin of the Hulk" in The Incredible Hulk #3 (Sept. 1962), Rosen became, with Artie Simek and, later, Rosen's brother Joe Rosen,[4][5] one of Marvel's primary letterers, hand-drawing the word balloons and sound effects for virtually every comic produced by the company, including such flagship titles as The Amazing Spider-Man and Fantastic Four.

[2] Rosen's last known credited comics work to appear is the 20-page story "Two Into One Won't Go" in Captain America #156 (Dec. 1972), which he was unable to complete.

Comics historian Nick Caputo reported that Rosen's last published work appeared in 1972, after which he "had a nervous breakdown".