Forbes was born in Riverhead, N.Y., and he graduated from the State University of New York at Albany. Forbes worked as an assistant for Sen. Al D'Amato and U.S. Rep. Connie Mack. In 1994, Forbes ran for the House of Representatives as a Republican. He defeated incumbent George Hochbrueckner by six percentage points. Forbes got a seat on the powerful Appropriations committee, unusual for a freshman representative, due to his ties with new House Speaker Newt Gingrich. In December 1996, Forbes announced he wasn't going to vote for Gingrich for speaker. Forbes voted for Rep. Jim Leach instead. Forbes became a pariah in his party for that vote. Forbes supported the impeachment of President Clinton.

On July 17, 1999, Forbes switched to the Democratic party. Forbes was promised financial support from House Minority Leader Dick Gephardt and Bill Clinton. Forbes became a target for the Republican party and Democrats who had run against Forbes were hostile. Forbes's only opponent was a 71-year-old librarian named Regina Seltzer. The Republican campaign committee chairman sent out videos of outtakes from Forbes's television commercials to Democratic voters in which he made flatulent noises with his hand under his armpit and mailings which were used to show him as an arch-conservative. Seltzer won a court ruling halting state Democratic party ads for Forbes. In the primary election, Seltzer defeated Forbes by 35 votes.