Hornberger's best-known work is his novel MASH (1968), based on his experiences as a wartime United States Army surgeon during the Korean War (1950–1953) and written in collaboration with W.C. Heinz.

It was used as the basis for an award-winning, critically and commercially successful movie – M*A*S*H (1970) — and two years later in an acclaimed long running television series (1972–1983) of the same title.

After graduating from medical school, he was drafted into the Korean War and assigned to the 8055 Mobile Army Surgical Hospital (M.A.S.H.).

"What characterized the fighting in Korea", one of Hornberger's fellow officers recalled, "was that you would have a period of a week or ten days when nothing much was happening, then there would be a push.

"[5] Hornberger's later assessment of his unit's behavior was: "A few flipped their lids, but most just raised hell in a variety of ways and degrees.

[7] After the war, Hornberger worked for the U.S. Veterans Administration, qualified for his surgical boards, and went into private practice in Waterville, Maine.

According to writer John Baxter, Hornberger "was so furious at having sold the film rights for only a few hundred dollars that he never again signed a copy of the book.

Hornberger reportedly did not like Alan Alda's portrayal of Hawkeye in the TV series, favoring the Robert Altman film, in which Pierce was played by Donald Sutherland.

While MASH was a fairly faithful reflection of Hornberger's service in Korea, his sequels were diverse representations of the "Swamp Gang's" post-Korea activities in the fictional town of Spruce Harbor, Maine, from 1953 to the 1970s.

The sequels are characterized by gentle humour, stereotypical local characters, and a nostalgic look at Maine and its people through Hornberger's eyes.

A series of novels based on the franchise was published in between M*A*S*H Goes to Maine and M*A*S*H Mania in which the characters travel to various locations, including Moscow, New Orleans, San Francisco, and Paris.