[citation needed] Hassan was elected to a total of three two-year terms, representing New Hampshire's 23rd district from January 2005 to December 2010.

She defeated former state senator Jacalyn Cilley in the Democratic primary and faced the Republican nominee, attorney Ovide M. Lamontagne, in the general election.

[20] In 1996, Hassan began working as an attorney for Sullivan, Weinstein & McQuay, a Boston corporate defense and business law firm.

[21] In 1999, then-New Hampshire Governor Jeanne Shaheen appointed her as a citizen advisor to the Advisory Committee to the Adequacy in Education and Finance Commission.

[citation needed] She served as the assistant Democratic whip, president pro tempore, and majority leader of the State Senate during her six years in office.

Larsen chose her for the position because she wanted someone who would fight to get the Democratic caucus to support the same agenda, at times creating friction between Hassan and her Republican colleagues.

[31] She won the Democratic primary with 53% of the vote, defeating former state senator Jacalyn Cilley, who received 39%.

[34] In the general election, Hassan defeated Republican nominee Ovide M. Lamontagne, 55% to 43%, carrying every county in the state.

[40] Hassan defeated Ian Freeman in the September 9 Democratic primary and Republican nominee Walt Havenstein in the general election, 52% to 47%.

[46] In response to New Hampshire's opioid crisis, she appointed Jack Wozmak the state's "drug czar" in early 2015.

[52] Hassan was endorsed by the pro-choice Democratic political action committee EMILY's List, which also backed her two gubernatorial runs.

[57] Hassan participated in a bipartisan Trump administration task force to support the reopening of the economy during the COVID-19 pandemic[58] Hassan was in the Senate chamber on January 6, 2021, for the 2021 United States Electoral College vote count when Trump supporters stormed the U.S.

A Hassan spokesperson confirmed that Marriott had been suspended from her position for a week and was required to return her congressional intern ID badge.

[67] Davis pleaded guilty to two misdemeanors in July 2019, acknowledging that she had given Cosko access to Hassan's Senate office after he was fired and had lied to investigators about it.

[75] In March 2018, Hassan was one of ten senators to sign a letter to Chairman of the United States Senate Committee on Health, Education, Labor and Pensions Lamar Alexander and ranking Democrat Patty Murray requesting they schedule a hearing on the causes and remedies of mass shootings in the wake of the Stoneman Douglas High School shooting.

[76] In October 2023, Hassan visited China as part of a bipartisan congressional delegation led by Senate Majority Leader Chuck Schumer and met with Chinese President Xi Jinping.

The delegation also met Foreign Minister Wang Yi, National People's Congress Standing Committee Chairman Zhao Leji, and Shanghai Communist Party Secretary Chen Jining.

[78] In July 2019, Hassan cosponsored the Fallen Journalists Memorial Act, a bill introduced by Ben Cardin and Rob Portman that would create a new memorial that would be privately funded and constructed on federal lands within Washington, D.C. to honor journalists, photographers, and broadcasters who have died in the line of duty.

[80] As of 2020, NORML, an organization that seeks legalization, gave Hassan a C− score as a U.S. senator due to her actions as governor.

[81] On February 3, 2021, Hassan announced she opposes raising the federal minimum wage to $15/hour as proposed in President Biden's American Rescue Plan legislation.

[82] On March 5, 2021, she and seven other Democratic senators voted with Republicans to block raising the minimum wage as part of the legislation.

[83][84] Hassan's husband, Thomas, was principal of Phillips Exeter Academy from 2008 to 2015, and as of 2016 is the president of School Year Abroad.

[6] In 2016, The Association of Boarding Schools censured Thomas Hassan for failing to disclose a former teacher's sexual misconduct at Phillips Exeter.