On the Republican side, John Kasich finished a surprising second, while Marco Rubio faded to fifth.
"We are going to make America great again," Donald Trump told packed Manchester victory rally, repeating his insurgent campaign's mantra.
Trump has dominated the Republican race for months, but time after time politicians and political observers questioned whether the New York billionaire could translate his populist appeal into electoral success.
There was no doubt about Trump's New Hampshire victory. The Associated Press called the race seconds after the polls closed. With about three-quarters of precincts reporting, Trump stood at 34 percent. No other Republican cracked 20.
"Do we have a ground game or what? You know, we learned a lot about ground games in one week, I have to tell you that," Trump said, alluding to his surprising loss to Texas Sen. Ted Cruz in Iowa, which many attributed to Cruz's superior organization.
Trump has tapped into Republicans' anger with Washington, D.C., and according to exit polls, he was on to something: An overwhelming 9 of 10 GOP voters say they're either dissatisfied or angry. According to those exit polls, Republicans said the economy, government and terrorism were their top issues of concern.
There's another, less official GOP winner tonight: Ohio Gov. John Kasich, who finished second in New Hampshire. That validates Kasich's decision to all but ignore the Iowa caucuses to focus on the Granite State.
"When the media kept saying, 'well, how are you going to do this? Can you finish high?' You know what I said?" Kasich told a crowd of supporters. "I have an insurance policy. It's you," he said, meaning his New Hampshire supporters.
Republican presidential candidate Ohio Governor John Kasich arrives on stage at a campaign gathering with supporters after placing second in the New Hampshire republican primary in Concord, N.H.
Andrew Burton/Getty Images
In an unconventional election, Kasich ran a very conventional New Hampshire campaign. He held more than 100 town halls and focused on a message of governance and compromise.
Trump had led by wide margins in New Hampshire for months, so the big question among observers was which so-called establishment candidate would emerge in second or third place to challenge Trump and Iowa winner Ted Cruz.
In the immediate days after Iowa it looked like Marco Rubio could take that prize. Crowds surged to hear the Florida Senator after his surprisingly strong third-place Iowa finish, and multiple high-level Republican officeholders endorsed him. But Rubio stumbled badly in Saturday night's debate and currently sits in fifth place, just behind Jeb Bush.
"I'm disappointed with tonight," Rubio told supporters Tuesday. "But I want you to understand something. Our disappointment is not on you. It's on me. I did not do so well Saturday night, so listen to this: that will never happen again."
It's clear New Hampshire did not deliver the clarity that the Republican establishment was looking for. Rubio and Kasich will carry on to South Carolina, as will former Florida Gov. Jeb Bush. Despite a fourth-place finish, Bush sounded resurgent Tuesday night. "The pundits had it all figured out on Monday night when the Iowa caucuses were complete," Bush said. "They said the race was now a three-person race between two freshman senators and a reality TV star. And while the reality TV star is doing well, it looks like you all have reset the race."
One Republican who might be thinking about calling it quits: New Jersey Gov. Chris Christie. Christie said Tuesday that rather than heading on to South Carolina, he'd head back to the Garden State to "take a deep breath" and reassess his campaign.
When Bernie Sanders began his presidential campaign last year, he told a crowd of supporters in Concord, N.H., Tuesday night, "we had no money, we had no organization, and we were taking on the most powerful political organization in the United States of America."
Tonight, however, he is the overwhelming winner of the New Hampshire Democratic primary. Sanders' big win comes a week after he lost the Iowa caucuses to Hillary Clinton by the narrowest of margins: just 0.3 percent.
"Together we have sent a message that will echo from Wall Street to Washington, from Maine to California," Sanders said. "And that is that the government of our great country belongs to all of the people and not just a handful of wealthy campaign contributors and their SuperPACs."
After Iowa and New Hampshire, it's clear the Democratic primary will now last much longer than most political observers ever anticipated.
"Here's what we're going to do," Hillary Clinton said during her concession speech in Hooksett, N.H. "Now we take this campaign to the entire country. We're going to fight for every vote in every state. We're going to fight for real solutions that make a difference in people's lives."
Democratic presidential candidate Hillary Clinton arrives onstage for her primary night gathering with daughter Chelsea Clinton and husband, former President Bill Clinton at Southern New Hampshire University in Hooksett, N.H.
Justin Sullivan/Getty Images
Clinton's campaign moved quickly to spin a narrow Iowa victory and double-digit New Hampshire loss, releasing a memo early Tuesday night framing the race as a long slog that will last, at minimum, through March.
While early states are important, campaign manager Robby Mook wrote, "the first four states represent just 4% of the delegates needed to secure the nomination."
"Whereas the electorates in Iowa and New Hampshire are largely rural/suburban and predominantly white, the March states better reflect the true diversity of the Democratic Party and the nation," Mook wrote. "Hispanics and African Americans play a critical role in who we are as a party and who we are as a nation." Both demographics will be much better represented in the next two states where Sanders and Clinton will compete: South Carolina and Nevada.
Younger voters overwhelmingly support Sanders, according to exit polls, but Clinton has long held an advantage over Sanders when it comes to popularity among minority communities.
Sanders' big advantage: online enthusiasm and the fundraising strength that accompanies it. "I'm going to hold a fundraiser right here, right now, across America," Sanders said Tuesday, urging supporters to donate at his website.
The Sanders campaign previously told NPR that it could raise between $30 million and $40 million in the days following a New Hampshire win.
Sanders' campaign manager, Jeff Weaver, conceded to NPR that Clinton has a big name-recognition advantage as the race broadens beyond Iowa and New Hampshire. But, he says, "Tonight was a critical night. We showed in Iowa we could go toe-to-toe with an establishment candidate, and tonight we showed the senator can win. And so I think for a lot of people around this country, it will give them an opportunity to take another look at him. Or maybe a first look."