With 99 percent of the precincts reporting, Coons had 57 percent of the vote to 40 percent for O'Donnell, an evangelical outsider whose stunning upset in the September GOP primary likely cost Republicans the race. Her opponent in the primary, congressman and former governor Mike Castle, had been considered a shoo-in to win Vice President Joe Biden's old seat.
When television screens showed Coons the projected winner soon after the polls closed, cheers erupted in a downtown Wilmington ballroom filled with Democrats.
"I'm honored and humbled by the confidence expressed by the voters of Delaware today, but now the hard work begins," Coons told The Associated Press. "I've said all along that this campaign is about Delaware's families and the challenges they face. Our job now is to see that Washington's focus is on jobs and getting our country back on track."
Coons, who has law and divinity degrees from Yale University, is executive of the state's most populous county, New Castle County, a Democratic stronghold that includes Wilmington. A wealthy attorney, he is the stepson of the founder of the company that developed Gore-Tex fabrics. During the campaign, he mostly supported the Obama administration's agenda, including the health care bill and the economic stimulus package.
Just a couple of months ago, he seemed an unlikely Democrat to withstand what was shaping up to be a tidal wave of Republican gains across the country. Coons, 47, won the nomination only after Biden's son, Attorney General Beau Biden, declined to run, making Castle the heavy favorite.
O'Donnell upended those plans by beating Castle in the primary, getting strong backing from tea party activists and a conservative base in the state's rural south. She also won key endorsements from Sarah Palin, the former Alaska governor, and influential Sen. Jim DeMint, R-S.C.
She quickly captured the national spotlight over quirky, evangelical comments she previously made on cable television shows such as "Politically Incorrect" with Bill Maher, where as a conservative activist she advocated chastity and equated masturbation with adultery.
She also faced questions about her background and personal finances, including inaccurate statements about her education, a tax lien from the IRS, a lawsuit from the university she attended over unpaid bills and a foreclosure action that she avoided by selling her house to her former campaign attorney before a sheriff's auction.
In her first ad, she tried to quell the firestorm surrounding her nomination and reintroduce herself to voters as a calm, mature leader. "I'm not a witch," she said in the ad, smiling. "I'm you."
But the ad sparked a fresh round of ridicule on comedy shows such as Saturday Night Live, and it became something of a rallying cry among her opponents.
"I'm sorry but I am not her," said Carol Terry, 72, an independent who voted at a Smyrna middle school. "She has no agenda, no experience."
Even Republicans had a hard time supporting her. The party establishment all but gave up on her campaign, and many GOP voters said they felt compelled to back Coons.
"I just couldn't see her as my senator," said Gary Stulir, a 41-year-old Republican from Smyrna. "She just couldn't take responsibility for anything she did ... I can't believe anybody backs her."
Still, with Delaware a possible bulwark against a Republican takeover of the Senate, Obama and Biden campaigned for Coons in October, and Biden headlined another rally Monday - despite polls consistently showing Coons with a double-digit lead.
Nearly six out of 10 Delaware voters approve of the job that President Barack Obama is doing, a contrast to Obama's job approval in many states, according to preliminary results of exit polls at precincts. Two-thirds have a favorable view of Biden, who represented Delaware in the Senate for more than three decades.
Coons' win could prove pivotal if Democrats attempt to move major legislation before the new Congress is seated in January. While other senators elected Tuesday will be sworn in then, Coons will be seated almost immediately to fill out the roughly four years remaining for Biden's old spot. The seat has been held temporarily by Ted Kaufman, a former Biden aide.
O'Donnell - who lost her third bid for Senate in five years - said she didn't know what she would do next. But in her concession speech, she said she was not going away.
"We worked hard. We had an incredible victory. Be encouraged. We have won. The Delaware political system will never be the same ... the Republican Party will never be the same," she said. Although she didn't blame the GOP establishment for her loss, she said the campaign would have done better if it had more support early on. She also dismissed criticism that she cost the party the seat by winning the primary.
"The people voted for me," she said. "People want their Republican leaders to represent the principles of the party."