During the Depression, he dropped out of school and worked at various jobs until he joined the Merchant Marine, before joining the United States Navy in 1942 for World War II. He served as a Seabee in a naval construction battalion.
In 1958, the 6-foot, 3-⁄2-inch (192 cm) (half an inch shorter than his co-star Clint Eastwood) Fleming landed the starring role as trail boss Gil Favor in Rawhide. Set in the 1860s, Rawhide portrayed the challenges faced by the men of the cattle drive from San Antonio, Texas, to Sedalia, Missouri. Producer Charles Marquis Warren called on the diary written in 1866 by trail boss George C. Duffield to shape the character of Favor: a savvy, strong, and fair leader who persevered and got the job done. The top-rated Western, with co-stars Clint Eastwood, Sheb Wooley, and Paul Brinegar, ran from 1959 to 1966. Fleming and Eastwood more or less rotated in playing the lead from week-to-week in the episodes, but Fleming was always billed over Eastwood.
After Fleming left Rawhide at the end of the 1964–1965 season (the series continued for only 13 more episodes before it was canceled by CBS), he took part in a Doris Day vehicle, The Glass Bottom Boat, playing a suave spy. He was then signed to star in High Jungle, a MGM adventure film shot in Peru. During a scene in the final stages of the film's production, Fleming and co-star Nico Minardos were in a dugout canoe that overturned in the Huallaga River. Minardos managed to swim to safety, but Fleming was swept away by the current and drowned on September 28, 1966. Fleming was only 41 years old when he died.
