Before his election to Congress, he also served as a member of Rep. F. Bradford Morse's Congressional Staff, and as a delegate to Republican National Conventions in both 1968 and 1972.

In 1972, he was elected as a Republican to the Ninety-third Congress, defeating future US Senator and Presidential candidate John Kerry, who had moved to the Fifth District to seek the seat after Rep. Morse resigned to take a post at the United Nations.

In the House, Cronin served on the Interior Committee, and began a process that led ultimately to the creation, years later, of an urban park in Lowell.

In 1974, his first bid for re-election, Cronin faced an assertive challenge from a Lowell-based county commissioner Paul Tsongas, who seized on President Nixon's impeachment troubles in what turned out to be a bad year, electorally, for Republicans nationwide.

Cronin later in life would serve a number of positions at Massachusetts Port Authority, and he unsuccessfully sought the GOP nomination for governor against William Weld.