He served in the United States Army for two years during the Korean War, then returned to the steel mill and enrolled at DePaul University College of Law, graduating with a Juris Doctor in 1957.

After law school, he opened a small law office in Chicago, Illinois, but soon returned to Massachusetts, where he spent two years as an Assistant District Attorney of Middlesex County and four years as an Assistant United States Attorney under Wendell Arthur Garrity Jr.

He remained in private practice until his appointment to the Superior Court of the Commonwealth of Massachusetts by Governor Michael Dukakis in 1974.

[3] In the early 1980s, the Conservation Law Foundation and the City of Quincy, Massachusetts sued the regional Metropolitan District Commission, saying that it violated clean water statutes because its antiquated sewage treatment plant on Deer Island was dumping hundreds of tons of black sludge into the harbor daily.

Judge Mazzone was known for his dedication not only to headline-producing cases, but to the routine caseload of a federal court, which he characterized as "doing the nation's work.