Bill Leahy attended the University of Notre Dame (1968) and Harvard Law School (1974). After practicing for 10 years as a trial and appellate public defender for the Massachusetts Defenders Committee, he was chosen as the first Deputy Chief Counsel for the Public Defender Division of the Massachusetts Committee for Public Counsel Services (CPCS) in 1984. In 1991, he became the second Chief Counsel of CPCS, leading that statewide public defender and assigned counsel agency until his retirement in July, 2010. He was lead counsel in the right to counsel case, Lavallee v. Justices of the Hampden Superior Court, 442 Mass. 228 (2004).

In February, 2011, Bill began his tenure as Director of the NYS Office of Indigent Legal Services, where he has undertaken the responsibility of improving the quality of representation for poor people in the criminal and family courts throughout the state.

From 2002-2011, Bill served as adjunct professor at Brandeis University, teaching courses in American Criminal Law and International Criminal Law and Human Rights. In December, 2012, he was a featured speaker at the International Conference on Criminal Legal Aid Systems in Beijing, China, where he presented a paper entitled An Assessment of the Right to Counsel in the United States

On March 15, 2013, he spoke as a panelist at the United States Department of Justice’s commemoration of the fiftieth anniversary of the famous right to counsel decision Gideon v Wainwright, 372 U.S. 335 (1963) (program viewable at http://www.justice.gov/atj/gideon/events.html), where he proposed the establishment of a National Center for Indigent Defense, and the creation of a White House Commission to advance the enforcement of the right to counsel. His paper Amplifying Gideon’s Trumpet, Revitalizing Gideon’s Rule: A Prescription for Action was published in the forthcoming online edition of the University of Louisville Law Review.

In 2018, Bill’s article, The Right to Counsel in the State of New York: How Reform Was Achieved After Decades of Failure was published in the Indiana Law Review