Ms Harris, 55, dropped out of the presidential race in December after failing in her bid to win the Democratic nomination.
She repeatedly clashed with Mr Biden during the primary election debates, most notably criticising his praise for the “civil” working relationship he had with former senators who favoured racial segregation.
The Democrat was born in Oakland, California, to two immigrant parents: an Indian-born mother and Jamaican-born father.
She went on to attend Howard University, one of the nation’s preeminent historically black colleges and universities. She has described her time there as among the most formative experiences of her life.
Ms Harris says she’s always been comfortable with her identity and simply describes herself as “an American”.
In 2019, she told the Washington Post that politicians should not have to fit into compartments because of their colour or background. “My point was: I am who I am. I’m good with it. You might need to figure it out, but I’m fine with it,” she said.
Sometimes the obvious pick is obvious for a reason. Kamala Harris was the front-runner to be Joe Biden’s running mate pretty much since the moment the presumptive Democratic nominee announced in March that he would pick a woman to be his ticket.
She’s relatively young and telegenic, and as the daughter of Jamaican and Indian immigrants reflects the growing diversity of the Democratic Party.
What’s more, she’s been in the spotlight of the national media, having campaigned for president in 2019 and, for a while last summer, risen to near the top of some polls. Many of her rivals for the number-two spot had never faced such scrutiny, so there was no proof that they could hold up under fire.
Another underrated advantage for Ms Harris was her friendship with Mr Biden’s late son, Beau, formed when they were both attorneys general. Mr Biden places a high value on family – and that connection may have made choosing her easier.
Now Ms Harris will have a chance to hit the campaign trail again and prove that she deserves this historic pick. If she succeeds, she’ll be in prime position to seek the presidency again, perhaps as early as 2024. Today has made her a force in the Democratic Party for years to come.
After four years at Howard, Ms Harris went on to earn her law degree at the University of California, Hastings, and began her career in the Alameda County District Attorney’s Office.
She became the district attorney – the top prosecutor – for San Francisco in 2003, before being elected the first woman and the first African American to serve as California’s attorney general, the top lawyer and law enforcement official in America’s most populous state.
In her nearly two terms in office as attorney general, Ms Harris gained a reputation as one of the Democratic party’s rising stars, using this momentum to propel her election as California’s junior US senator in 2017. She was only the second black woman ever elected to that chamber.
She launched her candidacy for president to a crowd of more than 20,000 in Oakland at the beginning of last year.
But the senator failed to articulate a clear rationale for her campaign, and gave muddled answers to questions in key policy areas like healthcare.
She was also unable to capitalise on the clear high point of her candidacy: debate performances that showed off her prosecutorial skills, often placing Mr Biden in the line of attack.
The self-described “progressive prosecutor” tried to emphasise more left-leaning parts of her legacy – requiring body cameras for some special agents at the California Department of Justice, the first state agency to adopt them, and launching a database that provided public access to crime statistics, though she failed to gain traction.
“Kamala is a cop” became a common refrain on the campaign trail, spoiling her attempts to win over the more liberal Democratic base during the primaries. Those same law enforcement credentials could, however, prove beneficial in the general election when Democrats need to win over more moderate voters and independents.
What’s the reaction?
President Trump told reporters: “She’s a person that’s told many, many stories that weren’t true.”
He added: “She did very, very poorly in the primaries, as you know, she was expected to do well and she ended up right around 2%. So I was a little surprised that he picked her.”
Mr Trump also said Ms Harris was “very, very nasty” and “horrible” to Mr Biden during the Democratic primary debates.
“She was very disrespectful to Joe Biden and it’s hard to pick somebody that’s that disrespectful,” he said.
The Trump campaign said the choice of running mate was proof that Mr Biden is “an empty shell being filled with the extreme agenda of the radicals on the left”.
Former US President Barack Obama – whom Mr Biden served as vice-president for eight years – tweeted: “She is more than prepared for the job. She’s spent her career defending our Constitution and fighting for folks who need a fair shake.
“This is a good day for our country. Now let’s go win this thing.”