William Williamson was born near New Sharon, Iowa on 7 October 1875 and moved with his parents to Plankinton, South Dakota in 1882. Williamson attended Wayne Normal School in Nebraska, and returning to South Dakota, he graduated from the University of South Dakota in 1903. Two years later he received a law degree from the same university and passed the South Dakota bar. Williamson began practice in Lyman, South Dakota, where he served as prosecuting attorney for Lyman County from 1905 to 1911. He served as the circuit judge of the eleventh judicial district from 1911 to 1921. In 1921, he successfully campaigned for the republican House of Representative seat in Congress. After 1933, Williamson returned to South Dakota and practiced law in Rapid City. He served on the Mount Rushmore National Memorial Commission from 1928 to 1972. Williamson died in Custer, South Dakota on 15 July 1972.
Congressman William Williamson was the driving force in getting money appropriated from Congress for the construction of the memorial. He was the one who convinced President Coolidge to come visit the Black Hills for a summer vacation in 1927. He also served as a member of the Mount Harney Memorial Society in 1925 until the Mount Rushmore National Memorial Society was created by legislation in 1929. He served as secretary, vice-president, and president of the Society