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 married to Helen V. Simpson.
He was a member of the New York State Assembly (Kings Co., 12th D.) in 1914, 1915, 1916 and 1917.
Simpson was again a member of the State Assembly in 1920; and a member of the New York State Senate (6th D.) in 1921 and 1922.
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.