Bill Cahn graduated from the University of Illinois and the University of Illinois College of Law and was appointed an assistant district attorney of Nassau County in 1950. During that tenure, he served as chief of the Homicide and Rackets Bureau and prosecuted many of the major criminal cases before the Court. In 1962 Bill was elected District Attorney of Nassau County and served for twelve-and-a-half years. During that time he was elected President of the New York State District Attorneys Association and later was elected President of the prestigious National District Attorneys Association. In 1971 the National Association voted him Prosecutor of the Year. He was also Vice President and lecturer at the National College of District Attorneys located in Houston, Texas.

In December 1971, Bill was selected alumnus of the month by the University of Illinois College of Law.

Bill argued three cases before the United States Supreme Court. He was successful in all three cases.