Born in Halifax, Nova Scotia, Foster worked as a staff artist for the Hudson's Bay Company in Winnipeg and moved to Chicago in 1919 where he studied at the Chicago Academy of Fine Arts and soon found illustration assignments. The illustrator J. C. Leyendecker was an early influence on Foster.

Foster's Tarzan comic strip, adapted from Edgar Rice Burroughs' novels, began January 7, 1929, continuing until Rex Maxon took over the Tarzan daily on June 10, 1929.

Foster returned to do the Tarzan Sunday strip beginning September 27, 1931, continuing until Burne Hogarth took over the Sunday Tarzan on May 9, 1937. He soon grew tired of adaptation and began planning his own creation.

William Randolph Hearst, who had long wanted Foster to do a comic for his newspapers, was so impressed with Foster's pitch for Prince Valiant that he promised Foster the ownership of the strip if he would start the series, a very rare offer in those days. The strip began February 13, 1937, continuing for decades. In 1954, Foster and his wife Helen were seen on television's This Is Your Life. In the late 1950s, the couple moved from Illinois to Redding Ridge, Connecticut. Later they moved to Winter Park, Florida. In 1967, Woody Gelman, under his Nostalgia Press imprint, revived some of Foster's earlier work.

In 1970, Foster was suffering from arthritis and began planning his retirement. He had several artists draw Sunday pages before choosing John Cullen Murphy as his collaborator and permanent replacement in 1971. Murphy drew the strip from Foster scripts and pencil sketches. Foster stopped illustrating (and signing) the Prince Valiant pages in 1975. For several years, he continued writing the strip and doing fairly detailed layouts for Murphy, eventually doing less and less of both the writing and art until Alzheimer's took his memory and he no longer remembered ever doing Prince Valiant.

Foster died in Winter Park in 1982.

Foster's impact on comic art was especially significant in the development of adventure strips, due to his controlled compositions, action scenes and detailed drawings of human anatomy and costume. His Prince Valiant pages were much admired by other artists, and he became a key influence on several generations of artists who were creating realistic comic strip and comic book illustrations, including Joe Kubert, George Pratt, Mark Schultz, William Stout, Warren Tufts, the British illustrator Bill Ward, Al Williamson and Wally Wood.

Foster was recognized for his work by the National Cartoonists Society with the Reuben Award in 1957, the Story Comic Strip Award in 1964, the Special Features Award in 1966 and 1967, all for Prince Valiant, the Elzie Segar Award in 1978, and the Gold Key Award (their Hall of Fame) in 1977. Foster was inducted into the Will Eisner Award Hall of Fame in 1996, and the Joe Shuster Canadian Comic Book Creators Hall of Fame for his contributions to comic books in 2005. The latter award was accepted on behalf of the family by an emotional Dave Sim, a modern comic writer and longtime fan of Foster's work.

Foster was inducted into the Society of Illustrators Hall of Fame in 2006.