Texas Congressman Ron Paul was a strong second, followed by former Utah Gov. Jon Huntsman Jr., early unofficial results reported by the New Hampshire Union Leader in Manchester showed. NBC News projected Paul had clinched second-place.
"Thank you, New Hampshire," Romney told his supporters at Southern New Hampshire University. "Tonight we made history."
Romney drew cheers when he called President Barack Obama "a failed president" and accused GOP rivals of joining forces with the Obama campaign to criticize his record as a venture capitalist. Without naming any Republican candidates specifically, he directed his criticism at what he called "some desperate Republicans."
"This is such a mistake for our party and for our nation," he said. "This country already has a leader who divides us with the bitter politics of envy. We must offer an alternative vision."
The staunchly libertarian Paul expressed pleasure with his push into second place after a strong third-place finish in Iowa, USA Today reported.
"I sort of have to chuckle when they describe you and me as being dangerous," Paul told his supporters gathered in Manchester. "That's the one thing they are telling the truth. We are dangerous to the status quo of this country."
Huntsman, addressing supporters after his third-place finish Tuesday, said, "Ladies and gentlemen, I think we're in the hunt."
"Hello South Carolina," Huntsman said, referring to the Jan. 21 primary in the Palmetto State.
Huntsman said Americans are "tired of being divided" and "no longer trust institutions."
With 72 percent of the precincts tabulated, Romney, the former Massachusetts governor who last week finished in a virtual tie with rival Rick Santorum in Iowa's non-binding caucuses, had 39.9 percent of the vote (76,384 votes).
Paul had 23.1 percent (44,223 votes) to 16.9 percent (32,415) for Huntsman.
In the next tier of the GOP field were former House Speaker Newt Gingrich of Georgia at 9.5 percent (18,227), Santorum at 9.5 percent (18,193), Texas Gov. Rick Perry at 0.7 percent (1,318), former Louisiana Gov. Buddy Roemer at 0.3 percent (568) and Fred Karger, a political consultant from Illinois, at 0.1 (173).
When the day's first votes were counted in the predawn hours in Dixville Notch, Romney and Huntsman tied at two votes apiece. Gingrich and Paul tied for second with one vote apiece.
President Barack Obama, running opposed, won all three votes cast in the Democratic primary.
Romney won the primary in slightly larger Hart's Location with five votes. Paul captured four votes, Huntsman two and Gingrich and Perry one each. Obama received 10 votes.
Hart's Location voting took 2 minutes, 26 seconds, the Union Leader reported.
Dixville Notch, an unincorporated village 20 miles south of Canada, has a population of 9. All nine residents voted.
Hart's Location, 70 miles south of Dixville Notch, has a population of 41 and 23 of them voted.
The communities are known for being among the first places to declare their results in the New Hampshire primary and general presidential election.