Mr Biden, a longtime senator from Delaware, is the 15th former vice president and second Catholic to become commander-in-chief.
The Democrat takes over command of the American executive branch from his 2020 election opponent, Republican Donald Trump, with a desire to hit the ground running to address the ongoing coronavirus pandemic and its resultant economic ruin.
The US crossed the threshold of 400,000 deaths from Covid on Tuesday, the eve of Mr Biden taking office.
Mr Biden’s 2020 running mate, Kamala Harris, was sworn in as vice president shortly before him.
Chief Justice John Roberts administered the oath of office to Mr Biden before a scaled-down crowd of close friends and family, former presidents, and members of Congress.
Associate Justice Sonia Sotomayor swore in Ms Harris.
Last week Mr Biden unveiled a $1.9trn legislative package complete with $1,400 stimulus checks, a $100 increase to the weekly, Covid-era federal unemployment benefit, billions of dollars in aid for state and local governments, and more.
Mr Biden has laid out a three-point plan of executive action for attacking the Covid crisis in his first 100 days as president:
Mr Biden has also pitched himself as a president for national “unity” and “healing,” promising to turn the page on the brash, often purposely divisive rhetoric emanating from the White House over the previous four years on everything from race relations and immigration to such democratic fundamentals as voting rights and basic government ethics.
That message of unity was expected to be the pillar of his inaugural address on Wednesday at the west front of the Capitol, which was a crime scene just two weeks prior as thousands of Trump supporters confronted and overpowered police on their way to ransacking the legislature.
Mr Trump, who was impeached by every House Democrat as well as 10 Republicans last week for inciting that insurrection, did not attend Mr Biden’s inauguration, breaking a 150-year tradition of living presidents welcoming their successors into the White House.
The now-former president and his first lady, Melania Trump, flew to Mr Trump’s Mar-a-Lago resort in Florida earlier on Wednesday.
His vice president, Mike Pence, represented the Trump administration at the inauguration ceremony.
Despite the open hostility of the now-former administration, Mr Biden has continued beating the drum of unity, a message he emphasised throughout his general election campaign and well into the post-election period.
“I will be a President for all Americans — whether you voted for me or not,” Mr Biden said during his victory speech on 7 November 2020.
Mr Biden is getting right to work on Wednesday.
Through executive actions, he will rejoin the World Health Organisation (WHO) from which Mr Trump withdrew the US last summer; rejoin the 2016 Paris Climate Agreement; repeal the so-called Muslim Ban barring travel to the US from several majority-Muslim countries; roll back several of Mr Trump’s anti-environmental initiatives; shore up protections for undocumented immigrants known as “Dreamers”; and rescind the 1776 Commission.
But while Mr Biden has laid out an ambitious schedule for his first 100 days, that agenda could be complicated — and perhaps overshadowed — by the uncertainty surrounding Mr Trump’s impending Senate impeachment trial.
Senate Republican Leader Mitch McConnell has not ruled out voting to convict Mr Trump of inciting the insurrection on 6 January that sent lawmakers fleeing for their lives, interrupting their proceedings to certify Mr Biden’s victory in the Electoral College.
A conviction vote — which would require at least 17 Republican senators to vote with all 50 Democratic senators — would bar Mr Trump from ever holding federal office again, choking off any speculation about a potential rematch with Mr Biden in 2024.
The insurrectionists who overtook the Capitol on 6 January were “provoked by the president and other powerful people,” Mr McConnell said in a speech on the Senate floor on Tuesday.
“They tried to use fear and violence to stop a specific proceeding of the first branch of the federal government which they did not like,” Mr McConnell said, adding that Mr Trump and others had “fed lies” to their followers about the 2020 election supposedly being “stolen.”
Mr McConnell was set to relinquish control of the Senate later on Wednesday with the swearing-in of Senators-elect Jon Ossoff and Raphael Warnock of Georgia and Alex Padilla of California.
The Kentucky Republican does intend to roll over and let Mr Biden assert his legislative agenda without a fight.
While he and Mr Biden agree on the basic fact that the newly sworn-in president won a free and fair election, they stay have drastically divergent views on everything from how to approach the Covid crisis, the US tax code, immigration reform, health care, and climate change to hot-button social issues such as abortion rights, gun control, and how to regulate Big Tech.
“November’s election did not hand any side a mandate for sweeping ideological change,” Mr McConnell asserted on Tuesday. “Our marching orders from the American people are clear: We’re to have a robust discussion and seek common ground. We are to pursue bipartisan agreement everywhere we can, and check and balance one another respectfully where we must.”
A fortress capital
The inauguration of a new president is traditionally a day for celebration, but an eerie quiet had settled over the capital city leading up to Wednesday’s ceremony with the memory of the Capitol riots two weeks early still fresh in everyone’s minds.
American democracy is “both fragile and resilient,” Republican Senator Roy Blunt of Missouri said on Wednesday in a speech at Mr Biden’s swearing-in.
Coast Guard boats with mounted machine guns patrolled the Potomac River.
More than 25,000 members of the National Guard — armed with black assault rifles and wearing full camouflage — stood watch over the nation’s capital before, during, and after the inauguration.
“There are more troops around the Capitol than there are in Afghanistan,” Senate Democratic Leader Chuck Schumer said on Tuesday in a speech on the chamber floor.
Everyone was on high alert for Mr Biden’s big day.
The Supreme Court received a bomb threat shortly after 10am on Wednesday. Several lawmakers wore body armour to the swearing-in ceremony.
Ms Harris was escorted onto the Capitol balcony on Wednesday by US Capitol Police officer Eugene Goodman, who has been hailed as a hero for diverting dozens of rioters away from the Senate chamber during the mayhem on 6 January.
A secure zone was established in downtown Washington for the ceremony at the Capitol and the subsequent presidential parade to the White House, with more than a dozen metro stations closed to deter people from travelling into the city.
The Guard had erected fences topped with barbed wire to surround the entire Capitol complex, where the inauguration ceremony commenced at 11am on Wednesday. Among the sacred buildings fenced off were the Capitol itself, the Library of Congress, and the Supreme Court, as well as the six House and Senate office buildings on the outskirts.
Barricades for vehicles extend even farther out: a three-block perimeter in every direction from the nearest vital federal building on Capitol Hill.
The Capitol itself had become a makeshift barracks for the thousands of troops stationed in Washington for the inauguration, with rows upon rows of cots lining the walls of hallways that in ordinary times are the frenzied highways of legislative business.
Trump bids farewell
While Mr Trump did not attend Mr Biden’s inauguration, he did wish his successor well in a videotaped “Farewell Address” released by the White House on Tuesday.
“We extend our best wishes,” the outgoing president said. “We also want them to have luck, a very important word.”
Mr Trump did not mention Mr Biden by name throughout the roughly 20-minute speech.
The former president hinted that he would be remaining active in American politics, indicating that the movement he started with his first presidential run in the summer of 2015 and promoted throughout the last four years was “only just beginning.”
Mr Trump touted the humming American economy before Covid sent it tumbling, his largely isolationist “America First” agenda overseas and the fact he had engaged America in “no new wars.”
He denounced the 6 January riots at the Capitol that he himself has been impeached for inciting and urged Americans to “rise above the partisan rancor.”
For Democrats in Washington, it was good riddance.
“Donald Trump was a stain on our country,” Speaker Nancy Pelosi, who did not personally speak to Mr Trump for nearly the last year and a half of his presidency, said in an interview with MSNBC released on Tuesday.
“I don’t think we could have sustained our democracy if he had had two terms in office for what he was doing to our institutions or what he was doing to our constitution,” the speaker said.
A new chapter for the world?
Democratic foreign policy leaders have been eager to reassert the US’ leading role in international affairs after Mr Trump spent four years openly antagonising the country’s NATO allies and straight up withdrew from several worldwide organisations and initiatives.
“America is back,” the president was fond of declaring in the post-election period.
Mr Biden stepped into office on Wednesday ready to “lead the world, not retreat from it,” he has said previously.
Democrats — and even many Republicans — know that no matter what Mr Biden does in the foreign policy realm over the next several months, they cannot simply paper over the damage to America’s standing in the world caused by Mr Trump’s fundamentally anti-democratic behaviour over the last two and a half months.
The president’s stonewalling of the Biden transition team, his constant lies about a stolen election during the transition, and his incitement of the mob that stormed the Capitol on 6 January has been a “substantial blow to US credibility abroad when it comes to promoting democracy and democratic values,” House Foreign Affairs Chairman Gregory Meeks told The Independent in a statement on Tuesday.
The deadly pro-Trump insurrection at the Capitol, where five people were killed, including a US Capitol Police officer, has already provided years-worth of ammunition for the propaganda machines of authoritarians and despots around the world who will “want to frame American democracy as frail, and [claim] that they can fill the void left by Trump’s isolationism,” Mr Meeks said.
Democrats’ No 1 job over the next four years will be working with Mr Biden and his administration to prove that the last four were “an aberration and not the new norm for the US role on the world stage,” the chairman said.