Mullins' New York Times obituary is here. But a more interesting link is here: besides many examples of Mullin's cartoons it includes a step-by-step set of photos showing how he worked.
Mullin apparently had little or no formal art instruction. That didn't stop him from gaining a good deal of knowledge about human and animal anatomy -- skeletal and muscular -- to be able to depict subjects both accurately or in hugely exaggerated ways. Many of his cartoons were of the exaggerated kind, but he fairly often would include a realistic portrait of a sports personality. These he usually derived from photographs using a pantograph. But he didn't slavishly trace his reference photos. Instead, as he once put it, he used the pantograph as a sketching tool.
Although he worked in other places on his way to sports fame, Mullin's best-known work was done for the New York World-Telegram evening newspaper, those cartoons usually focusing on New York City teams.
Click on the images below to enlarge.