He was born Charles Willard McLaughlin in Morrisburg, Ontario. At an early age his family moved to Brooklyn, New York. After two years, they moved to Cedar Rapids, Iowa, where McLaughlin finished high school. His parents returned to Canada, but he went on to study at Georgetown University in Washington, D. C., where he became involved in student plays. Adopting the stage name Willard Mack, after graduation he took minor acting jobs for a few years and did Shakespearian repertoire. However, writing scripts was what he was most interested in, and his second effort, about the North-West Mounted Police, In Wyoming, was a commercial success and was later the basis for his film Nanette of the Wilds. Throughout his life, Mack frequently returned to Canada. Some of his other plays, including Tiger Rose and The Scarlet Fox, were set in northern Alberta.
In 1914 he made his acting debut on Broadway in a play he had written. Over the next fourteen years, he would write a further twenty-two Broadway productions, acting in ten of them and producing four. For a time, Willard Mack operated a stock company with actress Maude Leone. In the mid-1920s, he met an aspiring stage actress named Ruby Stevens hired as a chorus girl for his new play. Mack coached Stevens's acting and rewrote parts of the play to expand her role and then persuaded her to change her name to Barbara Stanwyck.
During his time on Broadway, Mack began writing for motion pictures, and although he performed in fifteen films and directed four, he was primarily a writer. At first he remained on the east coast but later moved to Los Angeles. A number of his plays were made into motion pictures, and between 1916 and 1953 he was involved with the writing of more than seventy film scripts. Starting out in silent film, he made his talkie debut as actor, director, and co-writer of the 1929 film Voice of the City. In 1933 he directed What Price Innocence?. He then wrote and directed Broadway to Hollywood, a backstage musical that spanned nearly five decades recounting the struggles of a vaudeville family.