Born in London, Ontario, Lockhart made his professional debut at the age of six when he appeared with the Kilties Band of Canada. At the age of 15, he was appearing in sketches with actress Beatrice Lillie. Lockhart was educated in various Canadian schools and at the Brompton Oratory School in London, England. He also played football for the Toronto Argonauts.
Lockhart had a long stage career; he also wrote professionally and taught acting and stage technique at the Juilliard School of Music in New York City. He had also written theatrical sketches, radio shows, special stage material, song lyrics and articles for stage and radio magazines.
He made his Broadway debut in 1916, in the musical The Riviera Girl. He was a member of the travelling play The Pierrot Players (for which he wrote the book and lyrics). This play introduced the song, The World Is Waiting for the Sunrise, for which Lockhart wrote the lyrics along with Canadian composer Ernest Seitz. (The song was subsequently made popular by Les Paul and Mary Ford in the 1950s.) He also wrote and directed the Broadway musical revue Bunk of 1926. He also sang in Die Fledermaus for the San Francisco Opera Association.
However, Lockhart is mostly remembered for his film work. He made his film debut in the 1922 version of Smilin' Through, as the Rector, but did not make his sound debut until 1934 in the film By Your Leave, where he played the playboy Skeets. Lockhart subsequently appeared in more than 300 motion pictures. He often played villains, including a role as the treacherous informant Regis in Algiers, the American remake of Pepe le Moko, which gained him an Academy Award nomination for Best Supporting Actor. He also played the suspicious Georges de la Trémouille, the Dauphin's chief counselor, in the famous 1948 film, Joan of Arc, starring Ingrid Bergman. He had a great succession of "good guy" supporting roles including Bob Cratchit in A Christmas Carol and the judge in Miracle on 34th Street. He is also fondly remembered as the Starkeeper in the 1956 film version of Rodgers and Hammerstein's Carousel. Playing a bumbling sheriff, he appeared in His Girl Friday opposite Cary Grant and Rosalind Russell. He also appeared in the movie The Sea Wolf (1941), adapted from the novel by Jack London, as a ship's doctor. His last film role was that of the Equity Board President in the 1957 film Jeanne Eagels.
On Broadway, Lockhart originated the role of Uncle Sid in Eugene O'Neill's only comedy, Ah, Wilderness!, and took over from Lee J. Cobb as Willy Loman, during the original run ofDeath of a Salesman.
He has two stars on the Hollywood Walk of Fame at 6307 Hollywood Boulevard, one for motion pictures and one for television.
Lockhart was the husband of Kathleen Lockhart, the father of June Lockhart and the grandfather of Anne Lockhart.
Lockhart died from a coronary thrombosis at the age of 65 in Santa Monica, California. He is buried next to his wife in Culver City's Holy Cross Cemetery, a final resting place for people who were of the Catholic faith.