He swore to preserve, protect and defend America to the sound of cheers and applause from former presidents both Democrat and Republican – though Donald Trump decided to break precedent by missing the event.
It came minutes after new Vice President Kamala Harris took her oath, too.
Mr Biden stressed the fairness of last November’s election result in the opening of his inaugural address by declaring: “This is democracy’s day. The will of the people has been heard and the will of the people has been heeded.
“We’ve learned again that democracy is precious, democracy is fragile and at this hour my friends, democracy has prevailed.”
Mr Biden promised to “press forward with speed and urgency” during a “winter of peril” to tackle the “once-in-a-century virus that silently stalks the country”, also vowing to confront white supremacy and terrorism.
He stressed his prevailing focus after a divisive election campaign will be on “uniting our nation”, adding: “With unity, we can do great things, important things – we can right wrongs.”
And he said he wanted to “make America once again a leading force for good in the world”.
Mr Biden urged people to “join forces, stop the shouting and lower the temperature”, for, he explained, without unity there will be “no nation, only a state of chaos”.
Speaking as he looked out on to the National Mall lit by a bright sunshine, Mr Biden continued: “Politics doesn’t have to be a raging fire destroying everything in its path.
“Every disagreement doesn’t have to be a cause for total war and we must reject the culture in which facts themselves are manipulated and even manufactured.”
Repeating a motif he used back in November when he was projected to win the election, Mr Biden promised to be “a president for all Americans”.
Winding up his address, the new president struck an optimistic tone, saying: “Together we shall write an American story of hope, not fear, of unity not division, of light, not darkness.”
He ended with: “May God bless America and may God protect our troops, thank you America.”
Lady Gaga, wearing a large dove broach on her top and clasping a golden microphone, had just performed a rousing rendition of the national anthem – and Jennifer Lopez followed with an “American musical selection”.
Former presidents Barack Obama, George W Bush and Bill Clinton attended the event at the Capitol – and Mr Biden was greeted by cheers and applause as he walked up to the stage.
Mr Biden shared a fist-bump with Barack Obama before the pair took their seats, and then a series of speeches got underway – all sharing a theme of unity.
But as the new president prepared to take the oath of office, Donald Trump was landing in Florida.
Mr Trump is the first outgoing president since 1869 to skip an inauguration ceremony, but departing Vice President Mike Pence was in the audience.
As the inauguration ceremony took place in a chilly Washington DC, where it was trying to snow, the White House was getting a deep clean that was set to cost $500,000 (£366,000).
Shortly before the ceremony began, Mr Biden declared on Twitter: “It’s a new day in America.”
Mr Trump gave a parting message before boarding Air Force One, telling a small group of supporters and family members gathered on the tarmac of Joint Base Andrews that “we will be back in some form”
“I wish the new administration great luck and great success,” he added, before boarding the plane, which took off to the booming soundtrack of Frank Sinatra’s My Way.
Mr. Biden is only the second Catholic to hold the office of president.
His team have already announced he will sign a series of executive orders reversing several of Mr. Trump’s policies, including on COVID -19, Climate change and racial inequality.
