A furor broke out in 1957 over the gore and violence in horror comics, which led to the creation of the Comics Code Authority and censorship. Atlas lost its distribution network and had to cut back to a fraction of its output. For Heck that meant a layoff of a year and a half, during which he drew model airplane designs. Lee rehired Heck to replace Joe Maneely, who had died in an accident. When Heck returned to Atlas, its name had changed to Marvel and the mix of titles had changed to romance and science fiction. He contributed dozens of stories to Journey into Mystery, Strange Tales, Tales of Suspense, and Tales to Astonish before they ever featured the heroes that made them famous, as well as obscure titles such as Love Romances, My Own Romance, Strange Worlds, and World of Fantasy.
Heck graduated to the name "Dandy Don" when he started drawing heroes such as Thor and Giant-Man. His greatest claim to Marvel fame, however, may have been his contribution to the birth of Iron Man. Along with editor Stan Lee, writer Larry Lieber, and cover artist Jack Kirby, he created the Armored Avenger in Tales of Suspense #39 and (with three short breaks) drew his adventures for almost three years. Heck gave Tony Stark his movie-star good looks by basing him on Errol Flynn.
Artistically, Heck's influence is most strongly remembered from Avengers. The ubiquitous Kirby had pencilled the first 8 issues, but Heck took over on #9 (October 1964) and remained the primary artist on Avengers until 1967. Before, he had always inked his own work, and he preferred to do so, but he adapted to the situation. He also began contributing covers on a regular basis. Along with Kirby and Steve Ditko, Heck was one of the three major Marvel artists in the 1960's. He drew almost every Marvel hero.
In the late 1960s Heck branched out. He worked on The Man from UNCLE for Western Publishing and on the Phantom strip for newspapers. In 1970 he began doing some work for DC as well as Marvel, where he still worked on titles such as Amazing Spider-Man and X-Men.
During the early 1970s Heck suffered through a series of illnesses. His output slowed but never stopped. By 1979 he worked almost exclusively for DC, although he returned to Marvel in 1988 for a few projects.
Heck died of lung cancer on February 23, 1995. He was known for his ability to depict glamorous places and people, and for his willingness to take on last-minute assignments.