In a broadcast career that spanned more than a half-century, Hugh Downs was one of the most versatile and durable personalities on television. A mainstay of American TV-watching rituals for generations, he held for years the world record for most time on air — more than 10,000 hours — before Regis Philbin officially surpassed him in 2004.

When Mr. Downs signed off in 1999, he was known to millions of TV viewers as the co-host with Barbara Walters of the ABC newsmagazine “20/20.” Before that, he had been a host of NBC’s “Today” show, a long-running staple of American mornings, and before that a sidekick to Jack Paar on NBC’s “The Tonight Show,” a long-running staple of late-night viewing.

He also hosted the game show “Concentration.” By the time he retired, one reporter noted, Mr. Downs had been on TV nearly as long as TV had been on.

He “represented the entire history of broadcasting,” Ron Simon, curator for television and radio at the Paley Center for Media in New York, said in an interview. “Whatever the format, he was that consummate, quintessential broadcaster who could adapt his style to what was needed.”

Mr. Downs died July 1 at his home in Scottsdale, Ariz. He was 99. A great-niece, Molly Shaheen, confirmed his death and said the cause was a heart ailment.

“From ABC News, around the world and into your home, the stories that touch your life, with Hugh Downs and Barbara Walters” was the line that introduced “20/20” during its longtime Friday night slot. But the Downs-Walters pairing, one of the best known and most successful on-air partnerships of its era, had begun years earlier on “Today.”

Mr. Downs, a host from 1962 to 1971, had noticed Walters’s talent when she was a writer on the show and helped promote her as a “ ‘Today’ girl.” Walters was propelled to fame, later in the role of evening news anchor, as one of the most unreservedly inquisitive interviewers on TV.

“Hugh and I had different personalities and different styles, yet we complemented each other,” Walters wrote in her 2008 memoir, “Audition.” “He was more contemplative and thought of himself as something of a philosopher. His questions during interviews were gentler than mine, but he never restricted me from asking what I wanted. In short, he was . . . one of the truest gentlemen I have ever known.”

Mr. Downs’s other noted partnership was with Paar, the “Tonight Show” host who succeeded Steve Allen. For five years, from 1957 to 1962, Mr. Downs was the show’s announcer, the role later held by Ed McMahon under host Johnny Carson.

Like “Today” and “20/20,” the evening variety program allowed Mr. Downs to explore and expound on his interests, which included aviation, oceanography, music and other topics. On one occasion, he held forth on the physics of water-skiing, a topic that the host did not find overly interesting.

“Hugh,” Paar said, “when you drown you’ll know the reason why.”

Mr. Downs appealed to audiences for the authenticity that he exuded — a quality that would become increasingly rare as other news personalities sought to enhance their personal brands. Once, Paar asserted on air that he never deliberately embarrassed his guests.

“Yes, Jack, you do,” Mr. Downs replied, the televised setting of their exchange not impeding his honesty.

Millions of viewers would remember Mr. Downs’s performance in “The Tonight Show” installment in 1960 when Paar stunned audiences, as well as his announcer, by walking off the set to protest the network’s earlier censorship of his joke involving the term “W.C.”

Without taking Paar’s chair — a move that might have been perceived as opportunistic — Mr. Downs simply finished the program. “Jack, come back,” he said, gazing earnestly into the camera.

“Before Hugh was through,” a Time magazine writer observed, “not a viewer in 5,000,000 could doubt that he had watched a masterful high-wire artist solemnly treading his dangerous way between Paar and NBC.”

“That was probably the hottest spotlight I’ve ever been in,” Mr. Downs once told the Chicago Tribune, “and it probably did me a lot of good careerwise.”

In the late 1950s and into the 1960s, Mr. Downs hosted “Concentration,” the popular game show based on the game with the same name. He recalled that there were times when he would appear on “Today” in the morning, host “Concentration” in the afternoon and then go on Paar’s show in the evening.

The style that distinguished Mr. Downs — intellectual and unhurried — did not appeal to all viewers. “I know I’m perceived as bland,” he once told an interviewer. But his fans found him trustworthy, the quality that defined his time on “20/20.”

He joined the show in 1978 at the request of ABC News executive Roone Arledge, shortly after its debut with co-hosts Robert Hughes, an art critic, and veteran magazine editor Harold Hayes. The performance by Hughes and Hayes, who had minimal broadcast experience, was widely panned.

After several years as a “20/20” correspondent, Walters joined Mr. Downs as co-anchor in 1984. With her probing questions and Mr. Downs’s likable fascination with the world, they made the show a top TV newsmagazine.

Mr. Downs called “20/20” the “pinnacle of everything I had done.” He interviewed celebrities including the Dalai Lama, theoretical physicist Stephen Hawking, the Rev. Billy Graham and politicians such as Barry Goldwater. He had a particular interest in mental health and interviewed actress Patty Duke about her bipolar disorder, pointedly and memorably asking her about her suicide attempts.

Calling on his fascination with science, Mr. Downs traveled to the South Pole, submerged himself in the waters off Australia for a caged encounter with a great white shark and covered the story of Koko, the hand-signing gorilla.

While reporting on killer whales, he took a ride on one. Another time, he boarded the “vomit comet,” the NASA aircraft that simulates the weightlessness of space. With “20/20” correspondent Geraldo Rivera, he went diving in the Caribbean for a centuries-old Spanish galleon.

“Covering adventure stories let me be more than a reporter,” he said during a “20/20” tribute marking his retirement in 1999. “I became a cowboy in the great West in search of wild mustangs to tame.”

Mr. Downs was almost unfailingly agreeable and had an excellent rapport with Walters, despite their markedly different styles. He did, however, sit out the installment of ­“20/20” in 1997 when, over his objections, Walters interviewed sportscaster Marv Albert about his much-publicized sexual conduct.

“It was well produced and Barbara did a fine job,” Mr. Downs told Television Quarterly, according to the Atlanta Journal-Constitution. “But what did that do for people? It seemed pure tabloid, and I just couldn’t go with it.”

“Responsible journalism,” he continued, “helps enlighten.”

Mr. Downs remained on “20/20” for so long that the host who had once seemed fatherly began to appear grandfatherly. When he had a double-knee replacement in 1996, he documented the ordeal for his “20/20” viewers — missing only one night of his hosting duties.

Hugh Malcolm Downs was born Feb. 14, 1921, in Akron, Ohio. His father sold automobile accessories. The younger Mr. Downs attended what is now Bluffton University in Ohio and, in need of funds, found work in 1939 as a radio announcer in nearby Lima. He gradually moved up the ranks.

Mr. Downs moved to larger radio stations in Detroit, where he studied at Wayne State University, and later in Chicago. He said that when he did his first TV audition, in the mid-1940s, he had never before seen a television show.

One of his first TV jobs was with the “Kukla, Fran and Ollie” puppet show. By 1954, he was in New York, where he worked briefly with host Arlene Francis on the “Home” program.

Mr. Downs confessed that he initially dismissed television as a “gimmick.” The medium proved more popular than he anticipated, however, and he got his first prominent TV gig as an announcer on “Caesar’s Hour,” the NBC comedy sketch show featuring Sid Caesar, during its last season on the air. The next year, he joined “The Tonight Show.”

In 1944, Mr. Downs married Ruth Shaheen. She died in 2017. Survivors include two children, Hugh R. “H.R.” Downs and Deirdre Downs; a brother; two grandchildren; and four great-grandchildren.

True to the persona he presented to his audiences, Mr. Downs cultivated many interests outside television. He composed music and sailed a boat to Tahiti, a voyage that he chronicled in his 1967 book “A Shoal of Stars.” His other books included the memoirs “Yours Truly” (1960) and “On Camera: My 10,000 Hours on Television” (1986) and several books on the theme of aging gracefully.

“I personify the audience as they would like to be,” he once told Look magazine, “talking about many things with many people.”