Mr Trump, a Republican, declined to disavow a bizarre online conspiracy, while Mr Biden, a Democrat, would not divulge plans for the Supreme Court.
Both candidates were on the defensive at times on their records, Mr Trump for a range of issues and Mr Biden on race.
Opinion polls indicate Mr Biden has a solid lead over Mr Trump.
However, polling is still very close in several key states which could decide the election.
More than 18 million people have already voted in person or by post for the 3 November vote.
Thursday night’s town halls, as TV voter question-and-answer events are known in the US, replaced a cancelled second presidential debate. Mr Trump had refused to join that showdown virtually, following the recent Covid-19 diagnosis from which he now appears fully recovered.
What were the key moments for Trump?
During his primetime event hosted by TV network NBC in Miami, Florida, the president was asked about the QAnon conspiracy theory, whose adherents believe that Mr Trump is battling a clandestine network of elites, often involving Satanic plots and child trafficking.
When moderator Savannah Guthrie asked Mr Trump whether he would reject them, he replied: “I know nothing about QAnon.”