"I just received a very gracious call from Sara Gideon conceding the race," Collins told supporters in Bangor, Maine.
"I feel that this is an affirmation of the work that I'm doing in Washington to fight hard every day for the people of Maine," she continued. "Finally, let me say that this is an honor that is historical as well. I am the first person since Maine directly elected its senators to win a fifth term."
The Fox News Decision Desk projected Collins as the winner on Wednesday, which could keep the balance of power in the Senate in Republicans' favor.
"I have been humbled, touched and inspired by you and your stories each and every day," Gideon said at a news conference minutes after Collins' announcement. "Just now, I spoke with Sen. Collins. I congratulated her on winning this election, and I told her that I will always be available to help serve the people of Maine."
Collins, a perceived moderate, had taken sustained fire over the past few years due to her positions on various issues. One of the main drivers of progressive opposition to Collins is the fact that she voted in favor of Justice Brett Kavanaugh's confirmation to the Supreme Court in 2018.
She was one of the few Republicans who observers thought might buck the party line. Her decision helped place another conservative justice on the Supreme Court at a time when progressives accused Republicans of ignoring sexual assault allegations against Kavanaugh.
In addition to Kavanaugh's confirmation, however, Collins voted to pass President Trump's tax package in 2017. Despite the criticism over the two votes, Collins reportedly said she was "proud" of both actions.
The two candidates and their allies together have spent more than $120 million on television advertising alone.
Collins was battling for a fifth term against a candidate who drew donations from all corners of the country. Democratic support quickly coalesced around Gideon as Collins’ vote for Trump’s Supreme Court nominee Brett Kavanaugh ensured the race would draw national attention and cash.
Even without friendly fire, Collins was already facing tough odds as she was ran in a state that has repeatedly supported Democratic presidential candidates.
Although three of the six New England states — Massachusetts, New Hampshire and Vermont — have moderate Republican governors, 10 of the region’s 12 senators are Democrats, while Maine’s junior senator, Angus King, is an independent who caucuses with Democrats.
The Associated Press and Fox News' Fred Lucas contributed to this report.