[2] After graduating from North Quincy High School in 1972, he earned his bachelor's degree in history from the University of Massachusetts at Amherst and a Masters in Public Administration from Western New England College.

Morrissey earned a Juris Doctor from Suffolk University Law School in 1985 and was admitted to the Massachusetts Bar Association on December 17, 1985.

He was sworn in early, on January 2, 2011, to succeed William R. Keating, who had resigned from his position as Norfolk DA to serve in the United States House of Representatives.

[10] On Friday, August 25, 2023, District Attorney Morrissey's office released a pre-recorded video statement addressing concerns regarding public interest in the prosecution of Karen Read.

He cautioned citizens that online misinformation should never be “used as a pre-text to attack and harass others,” stating, “what is happening to the witnesses – some with no actual involvement in the case — is wrong...it needs to stop now.”[11] In the video, Morrissey refers to his message as "the first statement of its kind" in his twelve years as Norfolk District Attorney.