Following his graduation from Middlebury College in 1937, Heinz joined the staff of the New York Sun. After serving as the newspaper's war correspondent in Europe during the second world war, Heinz returned to the United States and was awarded his own sports column called "The Sport Scene," which primarily covered boxing, baseball, football and horse racing.

One of his pieces from around this time - Death of a Racehorse - is famous for its brevity (fewer than 1000 words) and its brilliance. The story centres on a promising young two-year-old horse racing for the first time, and concludes with the horse's death less than two hours later after it broke down in its first race.

Written in double quick time on a manual typewriter as the events unfolded, Death of a Racehorse is generally acknowledged as one of the greatest sports articles ever written and is referred to as the "Gettysburg Address" of sportswriting.

Heinz became a freelance writer after the Sun ceased publishing in 1950. He was a regular contributor to magazines such as SPORT magazine, Life, The Saturday Evening Post, Esquire, “True", "Collier's", and Look. The best of his magazine and newspaper pieces are published in his books "American Mirror" and "The Best of W.C. Heinz on Sports."

He published his first book in 1958, a novel called The Professional, the story of a young fighter pursuing the middleweight boxing championship. Ernest Hemingway called the book "the only good novel I've ever read about a fighter, and an excellent novel in its own right." Heinz edited two boxing anthologies, "The Fireside Book of Boxing" and "The Book of Boxing" with Nathan Ward.

Heinz's additional books include Run to Daylight with football coach Vince Lombardi, "The Surgeon", "Emergency," and "Once They Heard the Cheers," in which the author travels the country revisiting sports heroes of his past. In the late sixties Heinz collaborated with Dr. H. Richard Hornberger to write the novel M*A*S*H using the pen name of Richard Hooker. The book was the precursor to the film MASH which won the award for the best film of the 1970 Cannes Film Festival and an Academy Award for best screenplay based on another medium in 1971. The book also served as the prototype for the long running, Emmy Award winning television series.

A collection of Heinz's war writings including his dispatches from Europe and some post war articles were republished in his book, "Stories of World War II."

Heinz was a five-time winner of the E. P. Dutton Award for best magazine story of the year. He won the A. J. Leibling Award for outstanding boxing writing, and his work has been reprinted in more than 60 anthologies and textbooks. He was inducted into the National Association of Sportscasters and Sportswriters Hall of Fame in 2001 and into the International Boxing Hall of Fame in 2004. In 2008 the Associated Press Sports Editors posthumously awarded him the Red Smith Award for his contributions to sports journalism.

Heinz died in Bennington, Vermont, at age 93.