The former Massachusetts governor was considered the frontrunner going into the race, with the latest polls giving him 37 percent of the vote.
"Tonight, we made history," Romney said, speaking to his supporters in New Hampshire. He then wasted no time criticizing President Barack Obama, saying "this president wakes up every morning, looks out across America and is proud to announce, 'it could be worse.'"
He added, "The president is running out of ideas -- and now he's running out of excuses."
Meanwhile, CNN has projected that Rep. Ron Paul (R-Texas) will finish second and former Utah Gov. John Huntsman third.
With 19.3 percent of precincts reporting, Romney secured 35.5 percent of the vote, with Paul and Huntsman taking 24.7 percent and 17.0 percent respectively.