Joseph A. Trainor was born in Lawrence. He is a graduate of Saint Anselm College and Suffolk University Law School. After graduating from law school, he served as counsel and staff director of the Joint Committee on the Judiciary of the Massachusetts General Court. From 1978 to 1980, he was court administrator of the Juvenile Court Department of the Trial Court. From 1980 to 1985, Justice Trainor was general counsel and budget director of the House Committee on Ways and Means. In 1985 he joined the Boston office of Deloitte, Haskins & Sells as a senior manager, leaving after two years to establish his own consulting business. He returned to the State House in 1991, again as general counsel and budget director of the House Committee on Ways and Means. In 1992 Justice Trainor became general counsel of the Massachusetts Bay Transportation Authority. A year later he was named Assistant Secretary for Human Resources in the Executive Office of Administration and Finance and Chief Personnel Administrator for the Commonwealth.
Governor William Weld appointed him to the Juvenile Court in 1997; he served as first justice of the Middlesex Division for the next four years. Governor Jane Swift named him to the Appeals Court, where he took his seat on December 6, 2001. Justice Trainor has served on the boards of directors of numerous organizations and committees. He is also a disabled veteran of the United States Army.