William Stormont Hackett (December 7, 1868 – March 4, 1926) was an American lawyer, banker, businessman and politician. A Democrat, he was most notable for serving as the 70th mayor of Albany, New York after winning an election in 1921 that ended control of Albany by the Republican organization headed by William Barnes Jr., and established the dominance of the Democratic organization led by Daniel P. O'Connell and Edwin Corning.

A native of Albany, Hackett was raised in the South End neighborhood and attended the local schools. After two years at Albany High School, he left school in order to begin studying law at an Albany firm. After attaining admission to the bar in 1889, Hackett began to practice in Albany. He also served in the New York National Guard, and attained the rank of sergeant in Albany's Company A, 10th Battalion. In addition to practicing law, Hackett was involved in several business and banking ventures, including serving as president of the Albany City Savings Institution, Albany City Safe Deposit Company, and New York Mortgage and Home Building Company.

A Democrat who was friendly with local party leader Daniel P. O'Connell, in 1921, Hackett agreed to become the party's candidate for mayor. His defeat of William Van Rensselaer Erving ended Republican control by party leader William Barnes Jr. and ushered in O'Connell's longtime dominance of the city and county governments. Hackett was reelected in 1923 and 1925, and was planning to campaign for the Democratic nomination for governor of New York in the 1928 elections when he was injured in a February 1926 automobile accident that occurred while he was vacationing in Cuba. He did not recover, and died from his injuries at a hospital in Havana on March 4, 1926. A lifelong bachelor, Hackett was buried at Albany Rural Cemetery.

Hackett was born in Albany on December 7, 1868, the son of John Hackett and Martha Stormont. His family resided in Albany's South End neighborhood, and Hackett was educated in Albany. He began working while still a boy, including delivering the Albany Evening Journal newspaper, which was later owned by Republican Party leader William Barnes Jr.

After two years at Albany High School, Hackett dropped out in order to begin studying law in the office of Albany attorneys Amasa J. Parker Jr. and Edwin Countryman. He was admitted to the bar in 1889, and then practiced law in Albany.

Hackett also joined the New York National Guard, serving in Albany's Company A, 10th Battalion (formerly the Albany Zouave Cadets). He was a member of the unit from 1890 to 1899, and attained the rank of sergeant. Hackett later served as treasurer and president of Company A's veterans association.

A lifelong bachelor, Hackett was a member of the Baptist church, and became a lay preacher. He was also active in the Freemasons, and served as Master of Albany's Masters' Lodge Number 5 from 1900 to 1902. He was Master of the Albany chapter of Rose Croix, and Commander in Chief of the Albany consistory of the Scottish Rite Masons.

Hackett was a member of the Elks, the Fort Orange Club, the Albany Club, the Albany Country Club, and the Wolfert's Roost Country Club.