SAN FRANCISCO—Three years ago, Swati Mylavarapu had never worked for a political campaign and attended just a single campaign fundraiser.

Now, the 36-year-old Silicon Valley investor is a financial force behind one of the best fundraisers in the Democratic presidential primary, serving as national investment chairwoman for Pete Buttigieg, a fellow Harvard graduate and Rhodes Scholar whom she has known for half her life.

The campaign has raised about $76 million since the former South Bend, Ind., mayor entered the crowded race in early 2019—more than any Democrat except returning presidential candidate Sen. Bernie Sanders. Mr. Buttigieg’s financial structure sprouted from a supporter list of 24,000 emails and a handful of friends willing to help and now includes more than 733,000 campaign donors and about 125 people who each have raised at least $25,000.

Ms. Mylavarapu, who started her career at Google and worked at venture-capital firm Kleiner Perkins, brought a Silicon Valley mentality to the campaign, insisting that all donors and those who raise money be called “investors” rather than “bundlers.” (“We’re asking them to get out there and evangelize why they believe and to raise new dollars by sharing that story,” she said. “And what is that if it’s not investment?”)

Just as with some of his policies and campaign trail rhetoric, Mr. Buttigieg’s fundraising approach borrows from both the traditional and more liberal wings of the Democratic Party. Unlike Mr. Sanders, the Vermont senator, and Massachusetts Sen. Elizabeth Warren, he is holding fundraisers and pursuing $2,800 maximum donations, relying on Ms. Mylavarapu’s relationships with wealthy Democrats such as Esprit clothing company founder Susie Tompkins Buell.