Mr Warnock defeated Ms Loeffler in one of two runoff elections in Georgia that will decide control of the Senate, keeping alive Democrats’ hopes of taking control of the Senate.
The results will also determine whether Republicans will be able to block the agenda of President-elect Joe Biden.
There was still no clear winner in the race between Democrat Jon Ossoff and Republican David Perdue.
Mr Warnock was ahead of Loeffler by a percentage point or some 40,000 votes ahead of his rival, while Mr Ossoff led Mr Perdue by about 3,560 votes, according to Edison Research.
The Democrats would create history as no representative from the party has won a US Senate race in Georgia in 20 years.
Mr Warnock will become the state’s first black US senator while 33-year-old Mr Ossoff would be the Senate’s youngest member.
The Senate races drew 4.5 million voters – a new record for a runoff – as well as nearly £500million spent on advertising since November 3 and visits on Monday by both President Trump and Mr Biden.
The majority of votes there were left to be counted were in counties won by Mr Biden during the Presidential election two months ago, with around 30,000 to go in DeKalb and Newton counties near Atlanta, according to estimates from Eddison Research.
Speaking before he had been declared the winner, Mr Warnock claimed victory in a video livestream.
He said: “We were told that we couldn’t win this election. But tonight, we proved that with hope, hard work and the people by our side, anything is possible.
“I am going to the Senate to work for all of Georgia, no matter who you cast your vote for in this election.”
Ms Loeffler had not conceded the race was over when speaking at an Election Night Party in Atlanta.
She said: “It’s worth it for this election to last into tomorrow.
“We’re gonna make sure every vote is counted.
“Every legal vote will be counted. And I’m not gonna stop working,”
Gabriel Sterling, a top election official in Georgia, told NBC News it would be a tense race and that it would be a “long night.”
He said: “Its going to be a nickel and dime, scratching and clawing over the next several hours as these continue to come in.
“There’s big tranches for Democrats and there’s lots of little tranches of votes for Republicans.”
Election officials are taking a short break overnight but will rsume counting on Wednesday morning.
Georgia Secretary of State Brad Raffensperger told CNN: “Hopefully by noon we’ll have a better idea where we are.”
The Democrats need to win both of the votes in Georgia to take control of the Senate.
A sweep from the Republican’s rivals would create a 50-50 split in the Senate and give Vice President-elect Kamala Harris, as president of the Senate, the tie-breaking vot after she and Mr Biden take office on January 20.
If the Republicans manage to hold onto the second seat, they would effectively wield veto power political and judicial appointees made by Mr Biden, as well as several of the President-elect’s legislative initiatives in areas such as economic relief, climate change, healthcare and criminal justice.
This is a breaking story. More to follow…