Biden and Harris had arrived at the US Capitol on Wednesday for their official inauguration ceremony.
Biden, 78, was sworn in by Supreme Court Chief Justice John Roberts as the 46th president of the United States.
Harris, 56, is the first woman to become vice president.
Donald Trump left Washington three hours before Biden’s swearing-in, the first American president to skip the inauguration of his successor in more than 150 years.
The inauguration comes exactly two weeks after Trump supporters stormed the Capitol in a bid to disrupt the certification by Congress of Biden’s November election victory.
Among those attending the ceremony are former presidents Barack Obama, George W. Bush and Bill Clinton.
Vice President Mike Pence is also present after skipping a departure ceremony for Trump at nearby Joint Base Andrews earlier in the day.