Peter W. Agnes, Jr. grew up in Somerville, Massachusetts. He graduated cum laude from Boston University in 1972 and from Suffolk University Law School, cum laude, in 1975. He became a law clerk to New Hampshire Supreme Court Justice Edward Lampron in 1975. He then served as an Assistant District Attorney, first in Middlesex County where he became Chief of the Appellate Division, and then in Norfolk County, from 1976 through 1982. From 1986 to 1989 he was both the Assistant Secretary for Public Safety and Acting Director of the Massachusetts Criminal Justice Training Council and then served as Chief of Operations for Governor Michael Dukakis until 1991.
He began his judicial career as First Justice of the Charlestown District Court in 1991, advanced to the Superior Court in 2000, eventually becoming a Regional Administrative Justice in the Superior Court, where he was sitting at the time of his appointment to this court. Justice Agnes is a past president of the Massachusetts Judge's Conference and has served on and chaired a number of Supreme Judicial Court Advisory Committees and Massachusetts Bar Association Task Forces.
He is very active as an educator for both the courts and the bar, assisting organizations like the Flaschner Judicial Institute, MCLE, CPCS and various bar associations. He has also taught at the Massachusetts School of Law as an adjunct professor for the last 20 years. Appointed to the Appeals Court by Governor Deval Patrick, Justice Agnes joined the court in 2011.
Justice Agnes has chaired or served as a member of several Supreme Court Committees. He was appointed as the first chair of the SJC Standing Committee on Dispute Resolution in 1994 which was the first time a district court judge was appointed to chair an SJC Committee. In 2006 Justice Agnes was appointed as a member of the SJC Advisory Committee of the Law of Evidence. In 2015 he was appointed as the chair of that committee and as the editor-in-chief of the Massachusetts Guide to Evidence which has been published annually since 2008. In 2017, Justice Agnes was appointed the chair of the SJC Committee on Archives and Records Conservation. One of that committee's recommendation led to a agreement between the SJC and the Secretary of the Commonwealth which enables the SJC to store the judiciary's most historic permanent paper records in vault space at the state archives. In 2018, Justice Agnes served as a member of the SJC Committee on Best Practices before the Grand Jury. In 2019, Justice Agnes was appointed chair of the SJC Advisory Committee on Archives and Records.