With more than 90% of votes counted in Florida, Mr Trump, the Republican incumbent, was three points ahead of Mr Biden, the Democratic challenger.
But it is too early to project who won in other key states, including Georgia, Pennsylvania, Ohio and North Carolina.
The vote caps a long and bitter campaign amid the coronavirus pandemic.
More than 100 million people had already cast their ballots in early voting.
The US appears on course for its highest turnout in a century.