Bill de Blasio has emerged victorious in the race to succeed New York Mayor Michael Bloomberg, marking the first time a Democrat has captured City Hall in two decades.
De Blasio, the city's public advocate, beat Joe Lhota, his Republican rival, on Tuesday after a campaign in which he addressed economic inequality in America's most populous city.
"This election is a very stark contrast between two very different candidates. Mr. Lhota clearly wants to maintain the status quo in the city. I'm calling for fundamental change," de Blasio said after voting in Brooklyn on Tuesday morning.
De Blasio won a keenly contested Democratic primary in September by focusing on the controversial "stop-and-frisk" police tactic endorsed by Bloomberg and by criticising the billionaire mayor for presiding over "two New Yorks" - one rich, one poor.
In Boston, state representative Martin Walsh defeated city councilor and fellow Democrat John Connolly in a hard-fought race to succeed the city's longtime mayor, Thomas Menino.
Walsh was a union official before being elected to the Massachusetts House of Representatives in 1997.
In New Jersey, Chris Christie, a possible Republican presidential candidate, was re-elected governor with ease in one of the key races in Tuesday's off-year elections.
Christie is the first Republican in a quarter-century to get more than 50 percent of the vote in New Jersey, a state that voted overwhelmingly for President Barack Obama last year.
"A dispirited America, angry with their dysfunctional government in Washington, looks to New Jersey to say, 'Is what I think is happening really happening? Are people really coming together?'" Christie told supporters in his victory speech.
"Let me give the answer to everyone who is watching tonight. Under this government, our first job is to get the job done, and as long as I'm governor, that job will always, always be finished."
The off-year vote in several US states and cities will be scrutinised for clues to the mood of Americans ahead of next year's congressional elections - especially with a pragmatic conservative Republican, Christie, prevailing in New Jersey, while a more ideological one, Ken Cuccinelli in Virginia, lost a tight race to Democratic businessman Terry McAuliffe.
Setback for Tea Party
In Virginia, Cuccinelli's candidacy was hurt by his ties to the small-government Tea Party movement, which is widely blamed for causing last month's federal government shutdown.
The loss could be seen as a setback for the Tea Party, which swept a wave of conservative politicians into Congress in 2010 and has been looking to extend its influence in next year's congressional elections.
The Virginia vote drew intense national interest and more out-of-state funding than any gubernatorial contest in the state's history.
Major players of the Democratic party campaigned for McAuliffe in the final weeks, including Obama, former President Bill Clinton and his wife Hillary Clinton, a former secretary of state and possible 2016 White House contender.
President Obama phoned McAuliffe, as well as mayor-elects De Blasio and Walsh, all Democrats, to congratulate them Tuesday night on their election victories.
Obama vowed to work with them in the months ahead to expand economic opportunity for middle-class families.