Wellman's father, Arthur Gouverneur Wellman, was a New England Brahmin of English-Welsh-Scottish and Irish descent. william was a five times great-grandson of Puritan Thomas Wellman, who immigrated to the Massachusetts Bay Colony about 1640. william was a great-great-great grandson of Francis Lewis of New York, one of the signatories to the Declaration of Independence. His much beloved mother was an Irish immigrant named Cecilia McCarthy.

In World War I Wellman enlisted in the Norton-Harjes Ambulance Corps as an ambulance driver. While in Paris, Wellman joined the French Foreign Legion and was assigned on December 3, 1917 as a fighter pilot and the first American to join N.87 escadrille in the Lafayette Flying Corps (not the sub-unit Lafayette Escadrille as usually stated), where he earned himself the nickname "Wild Bill" and received the Croix de Guerre with two palms. N.87, les Chats Noir (Black Cat Group) was stationed at Lunéville in the Alsace-Lorraine sector and was equipped with Nieuport 17 and later Nieuport 24 "pursuit" aircraft. Wellman's combat experience culminated in three recorded "kills", along with five probables, although he was ultimately shot down by German anti-aircraft fire on March 21, 1918. Wellman survived the crash but he walked with a pronounced limp for the rest of his life.

Maréchal des Logis (Sergeant) Wellman received a medical discharge from the Foreign Legion and returned to the United States a few weeks later. He spoke at War Savings Stamp rallies in his French uniform. In September 1918 his book about French FLIGHT school and his eventful four months at the front, "Go Get 'Em!" (written by Wellman with the help of Eliot Harlow Robinson) was published. He joined the United States Army Air Service but too late to fly for America in the war. Stationed at Rockwell Field, San Diego, he taught combat tactics to new pilots.

While in San Diego, Wellman would fly to Hollywood for the weekends in his Spad fighter, using Fairbanks' polo field in Bel Air as a landing strip. Fairbanks was fascinated with the true-life adventures of "Wild Bill" and promised to recommend him for a job in the movie business; he was responsible for Wellman being cast in the Juvenile lead of The Knickerbocker Buckaroo (1919). Wellman was hired for the role of a young officer in Evangeline (1919), but was fired for slapping the leading lady, the Actress Miriam Cooper, who happened to be the wife of Director Raoul Walsh.

Wellman made his uncredited directorial debut in 1920 at Fox with The Twins of Suffering Creek. The first films he was credited with directing were The Man Who Won and Second Hand Love, released on the same day in 1923. After directing a dozen low-budget 'horse opera' films (some of which he would rather forget), Wellman was hired by Paramount in 1927 to direct Wings, a major war drama dealing with fighter pilots during World War I that was highlighted by air combat and FLIGHT sequences. The film culminates with the epic Battle of Saint-Mihiel. In the 1st Academy Awards it was one of two films to win Best Picture (the other was Sunrise), although, due to tensions within the studio regarding time and budget overages, Wellman wasn't invited to the event.

Wellman's other notable films include The Public Enemy (1931), the first version of A Star Is Born (1937), Nothing Sacred (1937), the 1939 version of Beau Geste starring Gary Cooper, Thunder Birds (1942), The Ox-Bow Incident (1943), Lady of Burlesque (1943), The Story of G.I. Joe (1945), Battleground (1949) and two films starring and co-produced by John Wayne, Island in the Sky (1953) and The High and the Mighty (1954).

Dorothy starred in Wellman's 1933 film Wild Boys of The Road and had seven children with Wellman, including actors Michael Wellman, william Wellman Jr., Maggie Wellman, and Cissy Wellman. His daughter Kathleen "Kitty" Wellman married actor James Franciscus, although they later divorced. His first daughter is Patty Wellman, and he had a third son, Tim Wellman.

Wellman initially worked fast, usually satisfied with a shot after one or two takes. And despite his reputation of not coddling his leading men and women, he coaxed Oscar-nominated performances from seven actors: Fredric March and Janet Gaynor (A Star Is Born), Brian Donlevy (Beau Geste), Robert Mitchum (The Story of G.I. Joe), James Whitmore (Battleground), and Jan Sterling and Claire Trevor (The High and Mighty). Regarding actors, Wellman stated in a 1952 interview, "Movie stardom isn't about acting ability - it's personality and temperament," and added, "I once directed Clara Bow. She was mad and crazy but what a personality!"

While he was primarily a Director, Wellman also produced ten films, one of them uncredited, all of which he also directed. His last film was Lafayette Escadrille (1958), which he produced, directed, wrote the story for and narrated. He wrote the screenplay for two other films that he directed, and one film that he did not direct, 1936's The Last Gangster. He also wrote the story for A Star Is Born and received a story credit for both remakes in 1954 and 1976.

Several filmmakers have examined Wellman's career. Richard Schickel devoted an episode of his PBS series The Men Who Made the Movies to Wellman in 1973, and in 1996, Todd Robinson made the feature-length documentary Wild Bill: Hollywood Maverick.

William Wellman, Jr. wrote two books about his father, The Man And His Wings: william A. Wellman and the Making of the First Best Picture (2006), and Wild Bill Wellman - Hollywood Rebel (2015). Wellman Jr. has been a guest-host on Turner Classic Movies to introduce films made by his father.

William Wellman died in 1975 of leukemia. He was cremated, and his ashes were scattered at sea. His widow, Dorothy Wellman, died on September 16, 2009, in Brentwood, California, at the age of 95.