With all precincts reporting, Clark took 31.6 percent of the vote in the seven-person primary in Massachusetts's 5th Congressional District. Middlesex County Sheriff Peter Koutoujian took 22 percent support.
Businessman Frank Addivinola took the Republican nomination, emerging victorious over physicist Mike Stopa and former congressional candidate Tom Tierney.
Clark had long been the front-runner in the race, nabbing the endorsement of EMILY's List and leading in most of the few polls conducted throughout the campaign.
She's expected to easily win in the Dec. 10 general election in a district that went with 65 percent for President Obama. Markey represented the seat since 1976, easily winning reelection for most of his career, until he gave up his seat to take the Senate seat vacated by Secretary of State John Kerry (D).