His first credited work for Marvel is The Incredible Hulk #3 (Sept. 1962). At one time or another, along with Artie Simek, Rosen worked on every series produced by Marvel in the 60's. Between them they did the lettering, word balloons and sound effects for almost every comic produced by the company. Rosen worked on flagship titles like The Amazing Spider-Man and Fantastic Four. In addition to his work for Marvel, Rosen was also a key letterer for Archie Comics.
Rosen's last credited comic is Captain America #156 (Dec. 1972). He passed away in 1992.