He was a Democratic member of the U.S. House of Representatives from Massachusetts from 1979 to 1993, and was the United States Ambassador to Trinidad and Tobago from 1994 to 1997.

During his tenure in Congress, Donnelly authored, along with Republican Congressman Bill Archer of Texas, legislation to repeal the Medicare Catastrophic Coverage Act of 1988 (Pub.

Donnelly's second major accomplishment in Congress was the enactment of the so-called "Donnelly Visa" program, which authorized 5,000 visas annually for citizens of countries that had been historically under-represented in the United States' immigration system that primarily relies on family reunification.

[3] As a Knight of Columbus, he helped defeat an effort to tax fraternal insurance companies which would have diminished their ability to make charitable contributions.

[9] Donnelly died from cancer at his home in East Dennis, Massachusetts, on February 28, 2023, just two days short of turning 77.