 Leavitt served on committees for reform of the Bar Association, where he proposed novel plans including reconstituting the United States Circuit Court of Appeals with a different composition of federal judges.John Brooks Leavitt was a member of the University Club, the Social Reform Club, the New York City League of Reform, the National Civil Service Reform League, the Barnard Club, Onteora Club, the Bar Association of New York City, the New York State Bar Association, the American Bar Association and the reform-minded Civil Service League.Leavitt was a long-serving member of the New York Bar Association's Committee on Unlawful Practice of the Law, as well as the Committee on the Prevention of Unnecessary Litigation.