After struggling financially during the Depression, Felsen sold nine books and hundreds of stories in his first eighteen months of full-time freelance writing in the early 1940s. After war service with the Marine Corps, during which he edited the Corps magazine Leatherneck and wrote magazine articles while stationed in the Pacific, he returned to Iowa where he lived for most of the rest of his life.

His best-selling book was Hot Rod, one of a rodding series that also included Street Rod and Crash Club and sold about eight million copies in all. He wrote about 60 books, many of them moralistically exploring the evils of drugs, sexism, and racism.

Felsen was born in Brooklyn, New York. He graduated from high school in 1933 and attended the University of Iowa for two years before dropping out due to financial difficulties. He found work in the Federal Writers' Project, producing articles for Iowa: A Guide to the Hawkeye State in 1936.

In 1937, he met and married Marie "Penny" Vincent from West Des Moines, Iowa, but the couple struggled through the remainder of the Depression. His wife found a position with Look Magazine, which was published in Des Moines, while he worked off and on for the Works Progress Administration, tried opening a fencing studio (which failed), and eventually became a full-time writer.

In 1940, Felsen started writing detective stories with Darrell Huff (an editor at Look Magazine). He then became a staff writer for David C. Cook Publishing Co. Shortly after that he published his first book, Jungle Highway. He quit that job after eight months to freelance in New York and over the course of the next eighteen months, he wrote and sold nine books and hundreds of stories.

A member of the Marine Corps for two and a half years during World War II, Felsen was stationed in the Pacific theater and wrote for Leatherneck magazine. He returned to Iowa in 1946 and remained there most of his life. From 1964 to 1969, Felsen taught writing part time at Drake University in Des Moines.

A prolific and popular children's writer, Felsen wrote about sixty books, with some of his most popular titles written from 1950 to 1960 in the series that included Hot Rod, Street Rod, Crash Club, Rag Top, and Road Rocket. He wrote the last book in the series, Fever Heat, under the pseudonym Angus Vicker. Hot Rod was so popular it remained on a bestseller list for children's books for thirty years, with multiple editions and more than eight million copies sold. Some of his other books became movies, while many of his stories appeared regularly in the Saturday Evening Post, another widely read national publication.