In an explicit play for young voters and Americans disillusioned by the state of the country’s politics, Robert F. Kennedy Jr. on Tuesday named Nicole Shanahan, a 38-year-old Silicon Valley lawyer, investor and political unknown, as his running mate in his independent presidential bid.

The selection of Ms. Shanahan brings youth and an outsider’s idealism to the ticket for Mr. Kennedy, the political scion whose long-shot White House campaign is threatening to complicate the electoral calculus for both President Biden and former President Donald J. Trump.

But it also brings a source of financial support: Ms. Shanahan, who was formerly married to the Google co-founder Sergey Brin, has already contributed more than $4.5 million to help Mr. Kennedy’s bid. Mr. Kennedy faces a costly effort to get on the ballot in all 50 states, as he says he will try to do. And he had a tighter timeline to name a vice-presidential choice, because some states require a full ticket to be on the petition for independent candidates to be placed on the ballot.

“I found exactly the right person,” Mr. Kennedy said at a rally in Oakland, Calif., as he revealed his choice. Praising Ms. Shanahan as a “gifted administrator” and a “fierce warrior mom” with experience navigating issues of artificial intelligence and intellectual property, he described her as “the daughter of immigrants who overcame every daunting obstacle and went on to achieve the highest levels of the American dream.”

Taking the stage more than a half-hour after Mr. Kennedy had announced her as his running mate, Ms. Shanahan echoed many of the themes of his campaign, including skepticism of vaccines, concerns about chronic disease and corruption in America, calls for a cleaner environment and the elimination of pesticides and genetically modified foods.

“The very failure of both parties to do their job, to protect their founding values, has contributed to the decline of this country in my lifetime,” said Ms. Shanahan, who described herself as a lifelong Democrat. “Maybe that’s why I see so many Republicans disillusioned with their party as I become disillusioned with mine. If you are one of those disillusioned Republicans, I welcome you to join me, a disillusioned Democrat, in this movement to unify and heal America.”

As recently as last month, Ms. Shanahan’s only public connection to Mr. Kennedy was her $4 million donation helping to pay for a Super Bowl ad backing him.

Though trained as a lawyer, her work in recent years has focused on funding research on health and the environment, issues that Mr. Kennedy — an environmental lawyer and vaccine skeptic who has promoted conspiracy theories — has made cornerstones of his campaign.

Ms. Shanahan took the stage on Tuesday more than two hours into the rally, after a lengthy parade of speakers who variously denounced school shutdowns during the pandemic, carcinogenic pesticides in the American food supply, the “corporate capture” of the American government and what they called censorship of dissident voices in the mainstream news media.

One speaker, Calley Means, the founder of a supplement and health company, said, “An unemotional statement of economic fact is that there has been no more profitable invention in the history of America than a sick child, a child with chronic disease.” Another speaker was Metta Sandiford-Artest, the former N.B.A. player who used to be known as Ron Artest and Metta World Peace.

Mr. Kennedy spoke alone onstage at length after naming Ms. Shanahan, saying she was going to stand up to Wall Street, “the crony capitalists” and Silicon Valley. He later said she was “going to help me free our country” from the “corrupt merger of state and corporate power that now straddles our nation’s capital like a mythical harpy.”

Ms. Shanahan first appeared to Tuesday’s audience in a prerecorded video, in which she described how her young daughter’s diagnosis with autism led her to research chronic disease and environmental toxins. Though she did not mention vaccines, this experience and her interest in the subject appeared to speak to a notable slice of Mr. Kennedy’s base: parents who are skeptical of childhood vaccinations and who criticize a medical establishment that they say pushes medical interventions on children.

When she took the stage, Ms. Shanahan at first appeared teary-eyed, but as she spoke, her voice grew steadier as she described her difficult childhood in Oakland. Her mother — who was in the audience — immigrated from China and worked as a caretaker and a secretary. Her father was often out of work, she said, and struggled with substance abuse.

In her remarks, Ms. Shanahan described a “crisis in reproductive health, embedded in a larger epidemic of chronic disease,” which she attributed to toxic substances in the environment, “electromagnetic pollution” — a reference that drew a loud and relieved “yes!” from a woman at the back of the cavernous event space — and medications. Chronic disease, she said, would be a major focus of her candidacy and a Kennedy White House.

Ms. Shanahan emerged as Mr. Kennedy’s favored vice-presidential pick in recent weeks, after the N.F.L. quarterback Aaron Rodgers and the former Minnesota governor Jesse Ventura had been at the top of his shortlist.

She has a history of contributing to Democrats, including Mr. Biden’s 2020 campaign. She has backed Mr. Kennedy since last spring, when he was still seeking the Democratic nomination. In October, he became an independent candidate, saying the Democratic Party had corruptly blocked his efforts to challenge Mr. Biden in the primary race.

In an interview last month, Ms. Shanahan said she had initially backed Mr. Kennedy because she was “excited” by him and was worried about Mr. Biden’s health. She said she had spoken with Mr. Kennedy only once, on the phone, when he was still running as a Democrat.

When Mr. Kennedy left the party, she was “incredibly disappointed,” she said, alarmed by how “divisive” she thought the move was. “I kind of withdrew, and paused all my political giving.”

It was only in January that she began to re-engage with Mr. Kennedy’s campaign, and as she did, she said, she found “almost like a secret society of individuals” who were backing him. “It was very, very interesting to me to hear how people have been kind of stirred by his message and his willingness to be out there,” she said.

She made the large donation helping to bankroll the Super Bowl ad supporting Mr. Kennedy, which was bought by an allied super PAC, and told The New York Times that she had provided creative guidance. The ad, which repurposed a 1960 ad for Mr. Kennedy’s uncle, John F. Kennedy, drew criticism from members of the Kennedy family, who have emphatically distanced themselves from the campaign.

Democratic allies of Mr. Biden have worried that Mr. Kennedy could tip the election to Mr. Trump, although polls have left a muddled picture of which major-party contender Mr. Kennedy would siphon more votes from. A recent Fox News national poll put his support at around 13 percent, drawing roughly equal shares of voters away from his dominant rivals.

Still, the Democratic Party has embarked on a legal offensive against Mr. Kennedy that aims to block him from the ballot, especially in crucial battleground states.

In a call arranged by the Democratic National Committee on Tuesday as a response to Mr. Kennedy’s announcement, Representative Robert Garcia of California said: “This is who Robert Kennedy selects, essentially a mega donor to fund his campaign with no experience, who is leaning into his anti-vax conspiracy theories, and has no real core as it relates to the truth or science.”

Allies of Mr. Trump also appeared wary of Mr. Kennedy on Tuesday.

A super PAC supporting the former president put out a statement describing Mr. Kennedy as a “far-left liberal with a far-left liberal running mate.”

Mr. Trump’s campaign spokesman, Steven Cheung, said later: “R.F.K. Jr. is a radical leftist — an environmental whack job who loves E.V. mandates, wants to end gasoline-powered engines. He’s no independent.”

Michael Gold contributed reporting