Jamie Kellner, a media executive who helped build Fox Broadcasting into a thriving television network with shows such as “Beverly Hills, 90210” and “The Simpsons” — and who went on to create the WB network, known for the angsty “Dawson’s Creek” and “Buffy the Vampire Slayer” — died on June 21 at his home in Montecito, Calif., near Santa Barbara. He was 77.

The cause was cancer, said Brad Turell, a family spokesman.

Mr. Kellner was one of the most successful television executives of his generation, whose knack for capturing young viewers — first men at Fox, then women at WB — lured viewers away from the Big Three networks that had ruled television for nearly 40 years.

Mr. Kellner believed ABC, NBC and CBS were ignoring viewers under 35 and were hamstrung by middle-of-the-road taste. Rupert Murdoch, Fox Inc.’s owner, and Barry Diller, its chairman, recruited Mr. Kellner from the television syndication business in 1986 and installed him as president of the Fox Broadcasting Company.

Its aspiration to be the first new TV network since ABC in 1948 was broadly derided. But from the debut in 1987 of its first series, the lowbrow family sitcom “Married … With Children,” which was shown on six Murdoch-owned stations and a string of independent ones that Mr. Kellner helped stitch together, the new network began stealing the Big Three’s audience.

By 1992, with shows like “Melrose Place,” about the social lives of 20-somethings, Fox was No. 1 with viewers 18 to 34. “We don’t really need anyone over 50 years of age to succeed with our business plan,” Mr. Kellner told The New York Times.

He resigned in 1993 after seven years at Fox. By then, Mr. Diller had left, and Mr. Kellner and Mr. Murdoch had clashed over Mr. Murdoch’s desire to pivot to older viewers and more mainstream shows.

Within months, Mr. Kellner was conjuring up WB, officially the Warner Brothers Network. He brought with him former Fox colleagues, including two rising programming executives, Garth Ancier and Susanne Daniels.

“He was a visionary in the television business,” Ms. Daniels, who went on to become president of MTV and head of original content at YouTube, said in an interview. Mr. Kellner “felt that Rupert Murdoch was making a mistake trying to, quote-unquote, ‘grow up’ the Fox network,” she added, “and that was an opportunity for the WB network to establish a strategy of attracting a younger audience who Fox was abandoning.”

Within just a few years, Tuesdays in prime time on WB, anchored by “Buffy the Vampire Slayer” and “Dawson’s Creek,” had become “a cult night on television for teenagers and 20-somethings,” Lawrie Mifflin wrote in The Times in 1998.

Although Mr. Kellner’s main job at the networks he built was to reel in advertising to pay for shows, and to corral affiliate stations to broadcast them, he could also be hands-on in encouraging promising writer-producers and in shaping content.

He helped ignite the careers of J.J. Abrams (“Felicity,” “Star Wars: The Rise of Skywalker”) Kevin Williamson (“Dawson’s Creek,” “Scream”) and Joss Whedon (“Buffy the Vampire Slayer,” “The Avengers”).

Mr. Turell, who was part of Mr. Kellner’s brain trust at WB, recalled Mr. Kellner suggesting to the producer Aaron Spelling (“Beverly Hills, 90210”) that a series about a preacher and his teenage daughter could capture the underserved audience of religious viewers. By that point, Mr. Kellner was himself the father of a teenage daughter.

The series that resulted, “7th Heaven,” with Jessica Biel, ran for 11 seasons and was WB’s highest rated show.

Mr. Kellner, who owned 11 percent of WB, cashed out after the network’s parent company, Time Warner, merged with America Online in 2000. He became chairman and chief executive of the new behemoth company’s Turner Broadcasting System, succeeding Ted Turner. Besides continuing to oversee WB, Mr. Kellner now also ran CNN and other properties. He moved from California to Atlanta, where Turner Broadcasting was based.

In a profile that year, the Times reporter Jim Rutenberg described Mr. Kellner as “square-jawed and street tough even though he now lives luxuriously.”

At CNN, which was struggling against the upstart cable news channels Fox News and MSNBC, Mr. Kellner rehired the financial anchor Lou Dobbs, brought on Anderson Cooper as a morning anchor and installed a respected journalist, Walter Isaacson of Time magazine, as chief executive.

But a rapidly shifting media landscape undermined some of Mr. Kellner’s ambitions. “Give us six months to a year,” he boasted in 2001, “we will be well ahead of Fox.” Roger Ailes, the head of Fox News, hung Mr. Kellner’s words in large letters as a taunt on the wall of his newsroom.

The changes Mr. Kellner brought to CNN did not arrest the onslaught of Fox, which carved out a niche with conservative viewers. A proposed merger of CNN with ABC News that Mr. Kellner favored was called off in February 2003.

That month, he announced he would step down when his contract ended and return to California. He retired from television at the age of 57 in 2004.

James Charles Kellner was born on April 18, 1947, in Brooklyn, one of five children of James Kellner, a commodities broker, and Jean (Mahan) Kellner, a librarian.

Early on, Jamie aspired to be a teacher. Eventually, however, he entered the TV industry through an executive training program at CBS.

He first struck programming gold in his mid-30s, teaming with Lorne Michaels, the creator of “Saturday Night Live,” to cut early episodes of “S.N.L.” to 30 minutes, then sell them in syndication to independent stations. It was that track record that led to his recruitment by Fox.

Mr. Kellner’s first marriage ended in divorce.

His survivors include his wife of 38 years, Julie Smith Kellner; their son, Christopher Kellner; a daughter from his first marriage, Melissa Kellner; two brothers, Thomas and Ronald;and three grandchildren.

In retirement, Mr. Kellner left the entertainment world behind for personal passions. He sailed his ketch, the Irishman, around the world, and started a winery, Cent’Anni, in the Santa Ynez Valley.