The Democratic duo, unveiled Tuesday by Biden after a months-long search for his partner on the ticket, now launch the 83-day sprint to the election face-off against President Donald Trump.
The choice of Harris, a 55-year-old US senator from California and daughter of Indian and Jamaican immigrants, was kept secret until the last moment, even though she was widely perceived as the wire-to-wire frontrunner.
In footage released Wednesday, Biden is seen speaking with her by video chat, apparently at the moment he told her of his decision.
"You ready to go to work?" he asks.
"Oh my God. I am so ready to go to work," Harris replies.
Biden's VP reveal marks a crucial moment as he aims to build a broad coalition of voters to defeat Trump.
It also offers a chance to capture momentum in a campaign turned upside down by the coronavirus pandemic, which has largely confined the 77-year-old Biden to his home in Wilmington, Delaware.
Harris, a former California attorney general, is the first woman of color and first Indian American to be a presidential running mate, and personifies the diversity seen as key to building enthusiasm for the Democratic ticket.
Scheduled to appear with Biden in Wilmington at about 4:00 pm (2000 GMT), Harris will be perceived by many voters as a safe pick, a known quantity vetted during her time as a presidential candidate and tested on the campaign trail.
She is also a fierce defender of efforts to end economic inequality and racial injustice -- in an election year marked by a historic reckoning on race.
She has valuable Capitol Hill experience and has shown her tenacity in holding Trump administration officials to account. And as California attorney general for six years Harris has run the largest statewide legal agency in America.
Biden has made clear he believes Harris has the policy proficiency and the governing experience needed for the White House.
"If @KamalaHarris and I are elected, we're going to inherit multiple crises, a nation divided, and a world in disarray," Biden tweeted Wednesday.
"We won't have a minute to waste. That's exactly why I picked her: She's ready to lead on day one."
Trump, who last month acknowledged Harris would be a "fine choice" for Biden, attacked her following the announcement as "nasty" and a radical leftist who would implement "socialized medicine" and confiscate Americans' guns.
But he and his campaign have appeared to struggle to formulate effective attack lines against a Democrat they long knew was a top VP contender.
Harris, who is two decades younger than Biden and Trump, could appeal to younger voters and women, particularly those in the suburbs who have been fleeing the Republican president, according to polling.
Trump sought to stake out his ground early Wednesday, insisting that "the 'suburban housewife' will be voting for me."
The next 11 weeks will see Harris pushing back against Trump, something she eagerly did on the Democratic debate stage when she branded him a "predator" of vulnerable Americans.
She has already attacked him over what she and Biden call Trump's failure to address the coronavirus crisis.
"In the middle of a pandemic, the president is trying to rip away health care. While small businesses close, he's giving breaks to his wealthy donors," Harris said in a video Wednesday.
"And when the people cried out for support, he teargassed them," she said, referring to June operations against demonstrators.
For Harris, being elevated onto a presidential ticket is the political moment of a lifetime. And if they win, the telegenic but tough politician would almost certainly become the 2024 or 2028 Democratic presidential frontrunner.
Biden would be the oldest person to take the office, and there is broad speculation that should Biden win, he would serve one four-year term and prepare his deputy to succeed him.
Biden has remained moderate in his political views for decades, while Harris is more progressive on issues like health care and climate policy.
Congresswoman Lisa Blunt Rochester argued that Biden and Harris are both pragmatic and progressive.
"Those are not mutually exclusive," she told MSNBC.
