His parents were Irish immigrants. At 17, his parents moved to East Berlin, Connecticut, and he became a laborer at American Iron Works, a job he continued when they moved to Northampton, Massachusetts. He happened to meet the actress Marie Dressler in 1902 and through her, went to New York to try for a career on the stage. He managed some burlesque and chorus-boy parts. In 1908, he began acting in Biograph films. His work there lasted until 1911; it included direction by D.W. Griffith and acting with Mary Pickford and Mabel Normand. By 1910, he was directing. In 1912, he and two bookies formed the Keystone production company. He brought Mabel Normand with him and soon added Roscoe 'Fatty' Arbuckle, Chester Conklin Al St. John, Slim Summerville, Minta Durfee, and Charles Chaplin (who was directed by Sennett in 35 comedies during 1914). He told Chaplin: "We have no scenario--we get an idea then follow the natural sequence of events until it leads up to a chase, which is the essence of our comedy." To the slapstick chase gags of the Keystone Kops were gradually added the Bathing Beauties and the Kid Komedies. In 1915, he and Griffith and Thomas H. Ince formed Triangle Films. Comedy moved from improvisational slapstick to scripted situations. Stars like Bobby Vernon and Gloria Swanson joined him. In 1917, he formed Mack Sennett Comedies, distributing through Paramount and later Pathe, launching another star, Harry Langdon. When he returned to Paramount in 1932, he produced shorts featuring W.C. Fields and musical ones with Bing Crosby. After directing his only Buster Keaton film, The Timid Young Man (1935) he returned to Canada a pauper. In 1937, he was awarded a special Oscar -- "to the master of fun, discoverer of stars ... for his lasting contribution to the comedy technique of the screen."