William Cowin graduated from Harvard College in 1959 and from Harvard Law School in 1962. He began his legal career as an assistant attorney general in Massachusetts until he became a legislative assistant to United States Senator Edward W. Brooke in 1967. Afterwards, he became special counsel to Mayor Kevin H. White of Boston. Justice Cowin also served as Secretary of Consumer Affairs and Secretary of Administration and Finance before joining the Boston law firm of Friedman & Atherton, LLP in 1975, later becoming partner. He practiced there for twenty-six years before being appointed to the Appeals Court by Governor Paul Cellucci in 2001. During his time on the Court, Justice Cowin authored over 400 opinions. He served as an associate justice until he reached the mandatory retirement age in 2008.
Currently, Justice Cowin is working part-time as a mediator at Boston's JAMS and as an assistant attorney general for Martha Coakley.