William Thorn Simpson (August 1886 – December 8, 1980) was an American lawyer and politician from New York.
Simpson was born in Patchogue, New York, and graduated from Patchogue High School and New York Law School.
He was a member of the New York State Assembly (Kings Co., 12th D.) in 1914, 1915, 1916 and 1917.
In June 1942, he was appointed as Chairman of the New York State War Ballot Commission.
He died on December 8, 1980, at the St. James Nursing Home in St. James, New York; and was buried at the Cedar Grove Cemetery in Patchogue.