The pair will make history together - he was the oldest ever President, she was both the first female and the first black and Asian American to be Vice President.
Branded a radical Left-wing socialist by Mr Trump, Ms Harris is so woke that she lists her personal pronouns - 'she/her' - in her Twitter profile.
Her mother was from India and her father was Jamaican. After they divorced, she was raised by her Hindu single mother, Shyamala Gopalan Harris, a cancer researcher and civil-rights activist.
'My mother used to have a saying, and she would say to me, "Kamala, you may be the first to do many things, but make sure you're not the last,"' Ms Harris recalled last year.
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Given Mr Biden's age, it is thought likely she will contest the next Presidential election for the Democrats in 2024.
A lawyer and former attorney general of California, she initially soared through a crowded field of Democratic candidates, only to drop out when her early gains faded.
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It was therefore a surprise when Mr. Biden announced that he had chosen her to take on Mr. Trump and his running mate, Vice President Mike Pence.
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In a volte-face, Ms. Harris tweeted: 'Joe Biden can unify the American people because he's spent his life fighting for us.'
She has taken a noticeably more moderate stance in recent months - being notably muted on calls by Black Lives Matter activists to defund police departments, arguing instead there should be 'reform' instead.
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Last year, she backed a Senate bill to legalize cannabis across America, adding: 'Times have changed - marijuana should not be a crime.'
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Ms Harris has also co-sponsored bills calling for tougher gun-control measures and 'Medicare for All' to establish a US health system open to everyone.
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While a rising star in Democratic politics for the past two decades, she was elected to the Senate only in 2016 - but has never shied away from her ambition.
'My mother always used to say, "Don't just sit around and complain about things. Do something,"' she tweeted after Mr. Biden announced that he had chosen her as his running mate.
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'I dearly wish she were here with us this week.'
Ms Harris went to Howard University, one of the top black colleges in America, and earned her law degree at the University of California.
She became the district attorney for San Francisco in 2003, before being elected the first woman and the first black person to serve as California's attorney general.
Ms Harris insists she has always been comfortable with her identity and describes herself simply as 'an American'.
In an interview with The Washington Post, she argued 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,' she said.
'You might need to figure it out, but I'm fine with it.'
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