By Carlos Greer
June 10, 2013 12:00 PM

Here’s how Tumblr founder David Karp, 26, celebrated the $1.1 billion sale to Yahoo! of the blog-hosting site he started in 2007: He took his mom and girlfriend to a casual brunch in his Brooklyn neighborhood, then to see the new Star Trek film. If it sounds entirely too low-key—how would you commemorate what amounts to an estimated $250 million windfall?—it isn’t as if he hasn’t ever splurged before. A year ago he bought an apartment for his mother, Barbara Ackerman, a science teacher who encouraged him to drop out of school at age 14 to pursue his passion for computers. “It’s the sort of thing that makes you feel so good it breaks your heart,” says Karp, chatting in a conference room at Tumblr’s open-space loft office in New York’s Flatiron district. “She tried to call to thank me, but it would have been too much. It’s hard not to get a little emotional when your mom tears up, particularly when it’s your fault.”

Yahoo!’s acquisition of Tumblr, with over 110 million blogs (including those of Lady Gaga and President Obama) and 175 employees, has been quietly in the works for seven months. It was hard on Karp, now among the tiny ranks of under-30 CEOs of billion-dollar tech companies, who is used to telling his parents everything. But the deal finally went public on May 20 with a statement from Yahoo! CEO Marissa Mayer, who praised Karp as “one of the most perceptive, capable entrepreneurs I’ve worked with.”

The tale feels familiar: Young genius makes good, despite dropping out of school. But where Steve Jobs (Karp’s hero: “I used to study his speeches”) and Mark Zuckerberg (the über-hoodie-wearing social media magnate) left college, Karp takes the legend further, moving it back to sophomore year of high school. Growing up on Manhattan’s Upper West Side, Karp was a curious kid who spent afternoons hanging out at his dad Mike’s music studio. “It was my favorite place in the world. There were mixing consoles and computers everywhere. I wanted to push everything!” He and younger brother Kevin, 21, attended the Calhoun School, an exclusive private school where their mom still teaches. “It was a small cohort of kids, and you feel like there are all these things you haven’t been exposed to,” recalls Karp. He transferred to the public Bronx High School of Science but spent most of his time teaching himself about building web programs. His parents, since divorced, hired tutors and allowed him to focus on computers full-time. “It was a really tough decision, but it was so clear that that’s what he needed,” says Ackerman.

He started interning with animator and mentor Fred Seibert, from whom he learned engineering at Seibert’s Frederator company. He later took a gig programming for the chatroom urban, which was eventually sold. He took his share of the earnings from the sale of that website to launch Tumblr from his bedroom.

There is no handbook for young men when their companies are suddenly worth a billion dollars. Shortly after the announcement, Karp flinched on national television when GMA anchor George Stephanopoulos called him a “hipster.” The host could be forgiven: Here’s a guy in a hoodie, who rides a motorcycle with sidecar and lives with his girlfriend of four years, NYU nursing student Rachel Eakley, 29, in a Williamsburg, Brooklyn, loft where former Gossip Girl star Penn Badgley and his girlfriend Zoe Kravitz, daugher of rocker Lenny, are their neighbors. On a credenza behind their sofa is a drawing of them on an island with the words “Hipster’s Paradise” scribbled underneath. It’s a joke; Karp rejects the cliché. “I just don’t know what it means to be a hipster. I never personally identified with it,” he says. Eakley admits, “I suppose we’re both on the chic-nerd-hipster spectrum.”

More important to him is navigating his new philanthropic opportunities; for this he is again consulting with Mom. “We’ve talked about Hurricane Sandy relief,” says Ackerman. “He has a huge heart and knows he can do some good.” Meanwhile his bulldog Clark is getting the good dog food, notes Eakley, and Karp is getting back to work. “We are laser focused to continue to deliver,” he says. “This is my heart and soul.”