The independent presidential candidate Robert F. Kennedy Jr. announced Nicole Shanahan as his running mate on Tuesday, elevating a little-known philanthropist and political donor to second-in-command on a ticket challenging President Biden and former President Donald J. Trump.

Here are three things to know about Ms. Shanahan.

She has signaled her support for Kennedy’s vaccine stance.

Ms. Shanahan told The New York Times in February that she had been drawn to Mr. Kennedy in part for his efforts to challenge scientific consensus on matters including vaccines.

Mr. Kennedy and his organization, Children’s Health Defense, have promoted debunked claims about the risks of vaccinations against measles, polio, tetanus, meningitis, Covid and other diseases.

“I do wonder about vaccine injuries,” Ms. Shanahan said last month, while saying she was “not an anti-vaxxer.” “I think there needs to be a space to have these conversations.”

She also praised Mr. Kennedy’s work as an environmental lawyer, though he has become better known for his anti-vaccine activism and his embrace of political conspiracy theories.

“I do think we have an environmental health crisis in this country,” she said. “I do believe Americans deserve clean water. And we can’t achieve that in the current climate of politics.”

She has never held elected office.

Ms. Shanahan has been a Democratic donor for over a decade but has never held or run for office. She has spent her career as a lawyer and tech entrepreneur focused on health and environmental research.

She founded and leads the Bia-Echo Foundation, which funds reproductive rights, criminal-justice reform and environmental projects, and previously founded ClearAccessIP, a patent analytics firm.

Ms. Shanahan married Sergey Brin, a co-founder of Google, in 2018, and they divorced last summer. Before that, she worked with the Sergey Brin Family Foundation, which funded a variety of left-leaning organizations.

She contributed $6,600 — the legal maximum — to Mr. Kennedy when he was running for the Democratic nomination last year, before he switched to running as an independent. She also gave $500,000 over the summer to Common Sense, a super PAC backing him. In 2020, she donated $25,000 to the Biden Victory Fund, following a donation of $2,800 to Marianne Williamson during the Democratic primaries.

She bankrolled Kennedy’s Super Bowl ad.

Earlier this year, Ms. Shanahan gave $4 million to American Values 2024, a super PAC supporting Mr. Kennedy, for the purpose of running an ad during the Super Bowl. She also helped coordinate production.

That contribution covered more than half the cost of the ad, which was nearly identical to one that John F. Kennedy, Mr. Kennedy’s uncle, ran during his 1960 presidential campaign. The remake angered some of Mr. Kennedy’s relatives, who criticized him for using images of — and Democratic nostalgia for — his uncle to promote a campaign that they argued the former president would have rejected.

A co-chairman of the super PAC, Tony Lyons, said last month that Ms. Shanahan had been “the driving force behind the decision” to remake the 1960 ad, after the group had to scrap an earlier ad idea because it showed Mr. Kennedy speaking directly to the camera and could have violated a ban on candidates’ coordinating with super PACs.

Rebecca Davis O’Brien contributed reporting.