After college, Trahan worked for Marty Meehan, member of the United States House of Representatives for Massachusetts's 5th congressional district, eventually becoming his chief of staff. In 2005, Trahan left the public sector to work for ChoiceStream, a Cambridge, Massachusetts-based marketing software company. She became the CEO of the Concire Leadership Institute, a small, woman-owned consulting firm.
In October 2017, Trahan announced her candidacy for the November 2018 election for the House to succeed retiring Representative Niki Tsongas. Tsongas had succeeded Trahan's former boss, Meehan, in a 2007 special election. In September 2018, Trahan won the Democratic primary election, the real contest in the Democratic district, after narrowly defeating Daniel Koh, the former Chief of Staff to Mayor of BostonMarty Walsh, in a field of 10 candidates. The victory was upheld following a recount. In the November general election, Trahan defeated her Republican challenger, Rick Green, garnering 62% of the vote.
Committee assignments
Tenure
Campaign finance investigation
On March 4, 2019, The Boston Globe published an analysis of contributions to Trahan's campaign in the weeks before the 3rd congressional district's primary where she beat Daniel Koh by less than 150 votes. In the last days before the primary, Trahan put hundreds of thousands of dollars into TV advertising and the Globe asked the question about the source of the money. Trahan offered an explanation to the Globe, claiming she used $371,000 in personal funds, but federal financial disclosures she filed in the late summer of 2018 appeared to show that she did not have the funds to cover such a campaign loan. In late March, the Campaign Legal Center asked the Federal Election Commission to open an investigation. In late April, the Foundation for Accountability and Civic Trust called on the Office of Congressional Ethics to investigate. On December 17, 2019, the United States House Committee on Ethics launched a continuing investigation of Trahan after congressional investigators found "substantial reason to believe" that she violated campaign finance laws in her election win in 2018.
In April 2019, Trahan said she supports the candidacy of the senior Senator from Massachusetts, Elizabeth Warren for president in 2020. In an Interview with WBZ-TV in April 2019, Trahan told the political commentator, Jon Keller, that she does not support the impeachment of President Trump, but said Congress should continue investigating the president. In December 2019, however, after the revelation that president Trump had spoken to the Ukrainian president about investigating his rival Joe Biden, Trahan told The Salem News that she supported the impeachment of the president saying Trump's "abuses in office a "clear and present danger" that requires action. On December 19, 2019, Trahan voted to impeach on both articles of impeachment against Trump.