John continued to draw, and took inspiration from such artists as Norman Rockwell, Alex Raymond, Hal Foster, and Milton Caniff.

In 1938, Rosenberger enrolled in night classes at the Pratt Institute,[3] where he met and started a romantic relationship with Marguerite "Peggy" Chapellier.

There, John taught at the U.S. Army Corps of Engineers School at Fort Belvoir, and was editor of the military magazine The Specialist, to which he contributed pin-ups and other illustrations.

In 1943 John was sent overseas for the remainder of the war, working on the construction of an oil pipeline in the China Burma India Theater.

After the war, the Rosenbergers moved into a new house in Jackson Heights, Queens, and John took a job at his father-in-law's gallery, eventually becoming manager.

Chapellier Galleries was located next to the Whitney Museum, and dealt in American art, endorsing such painters as William Merritt Chase, Robert Henri and Frank Duveneck.

Robert Bernstein was a writer Rosenberger met while drawing for Brevity, Inc., a company which produced industrial, political, and educational comics in booklet form.

Future newspaper strip pitches were less successful, with both Christopher Crown (1959) and Chris Cross (1965) rejected by the syndicates, but the two would become frequent creative collaborators on other projects.

After about a year in Connecticut, in which the Rosenbergers went deep into debt, John signed on for work with ACG and the family moved to Levittown, Long Island, close to their new good friends the Hugheses.

Cartoonist Fred Hembeck has noted that Rosenberger's superhero work showed his background in the romance genre, with "luscious babes" and a unique proficiency in rendering "expressions of impotent bewildered befuddlement" on the faces of male protagonists.

Around 1972–1974, Rosenberger was regularly working on various features for DC titles such as Strange Sports Stories, The Superman Family, Wonder Woman, and World's Finest Comics, and with such characters as Zatanna and Lois Lane.

He started penciling for Sy Barry, the regular artist for The Phantom newspaper strip, whose eyesight was beginning to fail him.