William Corrigan graduated from Cleveland Law School in 1915 and became an assistant prosecutor for Cuyahoga County the same year. In 1920 he decided to go into private practice. After initially working at another firm, he later assisted in forming the firm of Corrigan, McMahon, and Corrigan with his nephew Timothy McMahon and Joseph Corrigan (no known relation). Corrigan was a prominent defense and labor lawyer in Cleveland from the 1920s until his death. He is most widely known as the defense attorney for Dr. Samuel H. Sheppard in the Marilyn Sheppard murder case. While the murder cases he worked gained the most attention, he spent more time as a lawyer for labor unions—he was even named an honorary member of the International Ladies Garment Workers Union after serving those workers for many years. Other unions that frequently employed him as counsel include the Cleveland Federation of Labor and Building Trades Council.