He served on active duty in the United States Navy from 1955 to 1957 on destroyer escorts as the gunnery officer, and was a Lieutenant Junior Grade.

While at the Department of Justice, Harrington was a member of the special prosecution group conducting the nationwide probe of racketeering in the Teamsters Union.

During the so-called "long hot summer of 1964," Harrington was a member of a select team of attorneys dispatched to the State of Mississippi by Attorney General Robert Kennedy to protect the civil rights workers who were conducting "freedom schools" in voter registration there.

During this assignment, he was involved in the grand jury investigation of the murders of three civil rights workers in Philadelphia, Mississippi during that summer.

In that capacity, he participated in the successful prosecution and appeal of Raymond L. S. Patriarca, the alleged boss of the New England organized crime family, in 1968 for interstate racketeering.

The chief government witness in the Patriarca case, Joseph Barboza, was one of the first organized crime figures to break the "code of silence."

Teresa was the chief witness in 1971 before the Permanent Senate Subcommittee investigating organized criminal securities fraud in the Wall Street brokerage houses.

[4] In August 1977, Harrington was appointed by President Carter as the United States Attorney for the District of Massachusetts.

1981), a Petition for a Writ of Mandamus filed by the United States seeking the Court of Appeals to order the trial judge to recuse himself from the case.

Harrington helped shape the novel "fraud on the market" doctrine in security fraud cases, adopted the controversial use of "repressed memory" in sexual abuse cases, formulated the scholastic standards required of learning-disabled students in private schools, required standards for public school teachers and due diligence for federal regulators of the fishing industry, fashioned discovery rules for electronic documents, and upheld the supremacy of the cell-phone tower statute over local zoning regulations.

He participated in many major patent cases involving significant inventions in the medical, electronic, and communication fields, and applied the anti-trust theory to "buy-out" companies’ conspiring to depress the value of corporations intended to be acquired.