Born Michael Sinnott in Richmond Ste-Bibiane Parish, Quebec, Canada, he was the son of Irish Catholic John Sinnott and Catherine Foy, married 1879 in Tingwick St-Patrice Parish, Québec. The newlyweds moved the same year to Richmond, where John Sinnott was hired as a laborer. By 1883, when Michael's brother George was born, John Sinnott was working in Richmond as an innkeeper; he worked as an innkeeper for many years afterward. John Sinnott and Catherine Foy had all their children and raised their family in Richmond, then a small Eastern Townships village. At that time, Michael's grandparents were living in Danville, Québec. Michael Sinnott moved to Connecticut when he was 17 years old.
He lived for a while in Northampton, Massachusetts, where, according to his autobiography, Sennett first got the idea to become an opera singer after seeing a vaudeville show. He claimed that the most respected lawyer in town, Northampton mayor (and future President of the United States) Calvin Coolidge, as well as Sennett's own mother, tried to talk him out of his musical ambitions.
In New York City, Sennett became an actor, singer, dancer, clown, set designer, and director for Biograph. A major distinction in his acting career, often overlooked, is the fact that Sennett played Sherlock Holmes 11 times, albeit as a parody, between 1911 and 1913.
With financial backing from Adam Kessel and Charles O. Bauman of the New York Motion Picture Company, Michael "Mack" Sennett founded Keystone Studios in Edendale, California in 1912 (which is now a part of Echo Park). The original main building which was the first totally enclosed film stage and studio ever constructed, is still there today. Many important actors cemented their film careers with Sennett, including Marie Dressler, Mabel Normand, Charles Chaplin, Harry Langdon, Roscoe Arbuckle, Harold Lloyd, Raymond Griffith, Gloria Swanson, Ford Sterling, Andy Clyde, Chester Conklin, Polly Moran, Louise Fazenda, The Keystone Cops, Bing Crosby, and W. C. Fields.
Mack Sennett's slapstick comedies were noted for their wild car chases and custard pie warfare, especially in the Keystone Cops series. Additionally, Sennett's first female comedian was Mabel Normand, who became a major star under his direction and with whom he embarked on a tumultuous romantic relationship. Sennett also developed the Kid Comedies, a forerunner of the Our Gang films, and in a short time, his name became synonymous with screen comedy which were called "flickers" at the time. In 1915, Keystone Studios became an autonomous production unit of the ambitious Triangle Film Corporation, as Sennett joined forces with D. W. Griffith and Thomas Ince, both powerful figures in the film industry.
Also beginning in 1915, Sennett assembled a bevy of women known as the Sennett Bathing Beauties to appear in provocative bathing costumes in comedy short subjects, in promotional material, and in promotional events such as Venice Beach beauty contests.
Two of those often named as Bathing Beauties do not belong on the list: Mabel Normand and Gloria Swanson. Normand was a featured player, and her 1912 8-minute film The Water Nymph may have been the direct inspiration for the Bathing Beauties. Although Gloria Swanson worked for Sennett in 1916 and was photographed in a bathing suit, she was also a star and "vehemently denied" being one of the bathing beauties.