After hosting the local quiz show “It’s Academic” for 50 years, Mac McGarry, 85, has decided to retire. “I can’t tell you how much I’ve enjoyed doing the program,” McGarry says, “and I had no idea how pervasive it’s been to Washingtonians.”

For generations of Washingtonians, “It’s Academic” has been a soothing Saturday-morning time capsule, taped on a tiny set on Nebraska Avenue. It’s good pajama-day viewing — world capitals and world events and 11th-grade reading lists — and the centerpiece was always McGarry: paternal and chummy, dorky and suave, a comforting blend of Alex Trebek and Mr. Rogers, who wore tweed and pocket handkerchiefs and made the gentlest of jokes.

“Mac, Mac, Mac!” the lanky, teenage live audience would chant, with the kind of special affection reserved for the likes of Bob Barker and William Shatner, old guys who are, against all odds and logic, undeniably cool.

“One day I walked into the studio and [NBC Washington ­reporter] Pat Collins said to me, ‘Hey, Mac, whaddaya know?’ ” McGarry told The Washington Post for a story about the show’s 50th anniversary. “I said, ‘I’m the quizmaster. I know everything.’ ”

McGarry was a Channel 4 announcer in 1961 when he was assigned to work on a new teen program, patterned after the quiz shows that had recently become popular for adults. The original producers thought they’d be lucky to eke out a season or two. But over the years, McGarry became relentlessly devoted to the program, to the quest for knowledge, to the belief that teenagers were basically good. “He would demand that you call an embassy if there was a foreign word that he didn’t know how to pronounce,” says Susan Altman, the show’s producer. “He was always concerned about whether something was too passe, or would the kids still be learning that.”

McGarry, who lives in Potomac, had planned to return for the show’s 51st season, but he got a sinus infection this fall that made his voice raspy, and took some time off, then decided to retire permanently. “I wish all of the ­parents, teachers, students, drum majors — everyone who has supported us through the years — I wish them the best,” McGarry says. He still wants to be involved in the show in some capacity, he says. “Maybe I’ll write them some questions.”

Over the years, knowledge changed. Television changed. Washington changed. The man who knows everything has outlasted 10 U.S. presidents and seen current events become historical events. “It’s Academic” and Mac McGarry weren’t always captivating television, but they were reassuring television. He was someone you knew would be there.

McGarry will be replaced by WTOP news anchor Hillary Howard, who has been hosting on a temporary basis throughout the season. He may return for a farewell episode later in the season. “It’s impossible to state Mac’s impact on generations of Washingtonians,” Howard writes via e-mail, while on air. “They grew up watching, respecting and hoping to impress him with their fact retention and skill. They wanted Mac to like them. And he did.”