William learned to play lacrosse at St. Paul's School in Baltimore, and unbelievably played four years of varsity lacrosse in which the total record of the team was 59-0. Needless to say, the team won the MSA championship four years in a row (1943 to 1946). He was a second team All-Maryland player every year he played. He also was All-Maryland in football and basketball, and coached the St. Paul's JV lacrosse team in 1947.
Hooper then attended the University of Virginia from 1947 to 1951, and was captain of the varsity lacrosse team in 1949 and 1950. He was a second team All-American for two years (1948, 1949) and a first team All-American for two years (1950, 1951). Hooper played on the 1951 South All-Star team which lost 12-11 to a strong North squad despite his five assists.
He continued to play after college with the Mt. Washington Wolfpack and coached Towson State and Baltimore University. He officiated for three years, and also coached the South All-Star Team to a 12-9 victory in 1961. He was inducted into the Maryland Sports Hall of Fame in 1988.