As Biden acknowledged in his address, democracy is "fragile" and "precious".
It has prevailed, he said, but it "depends on all of us".
After the events of early January, the handover from Donald Trump to Biden cannot qualify as a peaceful transition.
Five people died in the assault on the legislative seat of government and the lives of congressmen and women were at risk.
Since then the immense wheels of power have turned to reclaim that territory and push back against a threat from within.
The US capital even established a Green Zone in its centre, which brought back memories of the one in Baghdad.
A hero of the Capitol siege, police officer Eugene Goodman escorted new Vice-President Harris at the ceremony.
Former presidents and top officials from both parties attended, closing ranks and sending pariah status chasing after the notable who didn't - now former president Trump. Many in Washington's political establishment will hope that cold shoulder becomes a permanent freeze.
It was left to outgoing vice-president Mike Pence - a target of the mob two weeks ago - to present a civil face from the Trump administration.
But beyond the optics, Biden's speech showed the 46th president is clear-eyed about the challenges ahead.
In a long plea for unity, he said the country must end "this uncivil war" and that Americans will need each other in this "dark winter".
He ran through the line-up of calamities the US faces and asked "are we going to step up?"
Biden and Harris now have the power, but also the pressure, to deliver.
Part of the strength on display was a return to the diversity of the Obama era. "We the people," once again means those of different heritage being prominently seen and heard.
Biden lacks the soaring rhetoric of former President Barack Obama but he has a refreshing, unadorned, directness that fits the moment.
And also a stream of optimism.
"Our better angels have always prevailed," he said.
As a self-described transition figure, the 78-year-old probably didn't mind being overshadowed momentarily by a 22-year-old.
Amanda Gorman delivered a poem that she finished after the Capitol assault.
"Being American is more than a pride we inherit. It's the past we step into and how we repair it," she said.
"There's always light if only we're brave enough to see it, if only we're brave enough to be it."