Rep. Michael Capuano (D-Mass.) conceded to Pressley on Tuesday evening.
Democratic U.S Congressional candidate and Boston City Councilor Ayanna Pressley greets voters outside a polling station in Boston, Massachusetts, U.S., September 4, 2018.
Pressley, who will not face a Republican challenger in November, becomes the first African-American woman elected to Congress from the Commonwealth.
Pressley called for defunding the federal Immigration and Customs Enforcement agency, known as ICE, which helped her draw support from Massachusetts' popular attorney general, Maura Healey, who's gained a national following for repeatedly suing President Donald Trump in an attempt to block his policies on immigration, gun control and other issues.
"Everyone knows she gave me my roots to my wings", Pressley told the Herald.
Pressley has bristled at the notion that race was a defining issue in her campaign.
But, she added, "When the challenges we are confronted with are this big, this deep, and growing, I can't and I won't wait my turn".
"This wasn't a battle between a conservative and a liberal, they were both progressives in very good standing", said Peter Ubertaccio, a professor of political science at Stonehill College outside Boston.
"These times demanded more from our leaders and from our party".
In a video clip of the moment, her family and staff can be heard shrieking and crying in the background as Pressley begins to hug those around her.
The last time an incumbent Democratic House member lost a primary was in 2014, when Seth Moulton defeated former Rep. John Tierney, who brought significant personal baggage to the campaign.
One longtime community activist in Boston, the Rev. Jeff Brown, spoke to the race's significance, saying it is "about the soul of the Democratic Party".