Hassan succeeds four-term Democrat John Lynch, who decided not to run for a fifth two-year term.
"Thank you to the people of New Hampshire for the trust you have placed in me," Hassan said during her victory speech in Manchester. "I will not let you down."
Hassan thanked Republican challenger Ovide Lamontage for his public service and said she will work with both sides of the Legislature to get things done.
"The challenges our state faces must be met with the best solutions and ideas we can muster," she said. "Good ideas and good people reside on both sides of the aisle."
The crowd at Hassan's election night party in Manchester erupted in cheers when the race was called.
The early call surprised those who closely watched this race. The AP called the race at 9 p.m., with Hassan ahead 59 percent to 38 percent, with 14 percent of precincts recorded.
The news had some at Lamontagne's election night party shaking their heads. Hassan campaigned in the final 48 hours with President Obama, and former President Bill Clinton, but Jack Kimball, a former state GOP chairman, said he doesn't think the numbers are a result of any presidential coattail effect.
If anything, it would be because of Governor Lynch's endorsement and campaigning on Hassan's behalf, according to Kimball, noting Lynch's popularity in New Hampshire.
Hassan had backing of several labor unions, and NH AFL-CIO President Mark MacKenzie was quick with a statement:
"Maggie Hassan's victory tonight should send a strong message to our elected leaders that New Hampshire workers want a governor who will continue in the footsteps of Governor Lynch. Maggie will govern based on the Granite State's needs, not a particular brand of ideology, and she will continue Governor Lynch's tradition of even-handedness and fiscal responsibility."
Lamontagne supported right-to-work legislation, which failed in the last legislative session in New Hampshire, while Hassan opposed it. Hassan argued that right-to-work would not benefit the state's economy.
Emily's List hailed the news, saying the pro-choice Hassan is now the only Democratic woman governor in the country.
Hassan, an attorney and former state senator, cast herself in the campaign as a leader in Lynch's image.
In debates, she described herself as a critical check-and-balance to Tea Party leaders in the Legislature. She said Lamontagne would do the bidding of House Speaker Bill O'Brien (R-Mont Vernon).
The race grew heated at times, as supporters of each aired various attack ads. Some ads called Lamontagne an "extremist" who would meddle with women's health care, not just opposing abortion rights. Another ad zeroed in on Hassan, classifying her as a tax-and-spend liberal who did not pay any property taxes on her house in Exeter – the ad failing to mention that her husband Tom, as principal of Phillips Exeter Academy, is required to stay on campus in the school-owned house.
Both candidates said job creation was their priority. However, the campaign season saw them trading barbs over social issues. Hassan supported the state's gay marriage law. Lamontagne did not.
Lamontagne warned of federal government overreach. He promised to do everything in his power as governor to block the Affordable Care Act, aka "Obamacare." Hassan, who fought for expanded insurance coverage as a state senator, said the issue showed a disconnect between Lamontagne and middle class families.
They found some common ground in support for expanded gambling, in the form of a highly regulated proposed casino in the southern part of the state, and support for medical marijuana.
Still, despite the broad range of issues debated over the past year, the two candidates kept bringing the debate back to pocketbook issues.
Hassan, 54, and her husband have two grown children.