After topping a crowded Democratic primary, the state senator ran a low-key general election campaign in a state where voters have grown weary of special elections.
Clark thanked family members and backers gathered in Stoneham, north of Boston, to celebrate the win. She credited her supporters for helping her launch the campaign in the heat of the summer and seeing it through on a snowy, slushy final push Tuesday.
"And you know what? It was all worth it," said Clark, who could be sworn in as early as this week to represent the state's 5th Congressional District.
Clark credited her win in part on her campaign's ability to connect with voters.
"We had a message that resonated," she said. "It's time for Congress to get back to work for families."
Turnout was light across the district with the candidates competing for the attention of voters, many of whom were in the midst of holiday preparations.
Republican Frank Addivinola, a Boston attorney, was Clark's closest opponent. The other two candidates were James Aulenti and James Hall.
The 5th District is heavily Democratic and overwhelmingly backed Barack Obama over former Massachusetts Gov. Mitt Romney in last year's presidential contest. The district stretches from the coast to communities north and west of Boston including Waltham, Framingham and Medford.
The seat became vacant earlier this year when longtime Democratic U.S. Rep. Edward Markey won a special election to fill the Senate seat left open after John Kerry was named secretary of state.
Clark will fill out the remainder of Markey's two-year term and face re-election next fall.
Clark and Addivinola offered voters a stark choice on issues ranging from abortion to the federal health care law.
Clark supports abortion rights and said that while she's disappointed with its website problems, she believe Obama's health care initiative is a historic law that could pave the way to even broader health care coverage for Americans.
Addivinola opposes abortion and believes that an unborn fetus is entitled to legal protections. He also criticized the federal health care law and said efforts to expand health coverage are best left up to states.
Clark, 50, said her priorities also include ending gun-related violence, increasing the minimum wage, supporting Social Security and early education and guaranteeing pay equity for women. Addivinola, 53, had described himself as a "small government kind of candidate" and blamed a lack of leadership in Washington for the nation's stubborn unemployment and underemployment rates.
Clark also enjoyed a fundraising edge in the race, receiving nearly $1.2 million in political contributions through Nov. 20. She also poured an additional $250,000 of her own money into her campaign. Addivinola collected just $38,334 in donations during the same period and contributed more than $61,000 of his own money to his campaign.
Clark won the backing of big-name Democrats during the campaign including House Democratic leader Rep. Nancy Pelosi of California, and Massachusetts' two U.S. senators, Elizabeth Warren and Markey.
Clark, a lawyer and former public interest attorney who was first elected to the Massachusetts Legislature in 2008, enters Congress as the newest member of a minority party in a chamber where Democrats have few levers of power.
The only other women representing Massachusetts in the U.S. House, Democrat Niki Tsongas, congratulated Clark.
"Six years ago, there wasn't a single woman representing Massachusetts in Congress," Tsongas said, also noting Warren's election last year. "As Katherine Clark joins me in the House of Representatives this week, Massachusetts gains another strong voice to its increasingly diverse Congressional delegation."