Sid Mark, 88, the radio DJ who entertained generations of listeners in Philadelphia and across the country playing the music of Frank Sinatra for more than 65 years, has died.

Mr. Mark’s death on Monday was confirmed by David Heim, a communications manager for Audacy, the Philadelphia-based company that owns WPHT-AM (1210), the Philly radio station that carried Mr. Mark’s Sundays With Sinatra program. A cause of death was not given.

Mr. Mark’s longevity — and his loyalty to Sinatra — was legendary. In addition to Sundays With Sinatra, he had a long-running Philadelphia show called Fridays With Frank and a nationally syndicated program, Sounds of Sinatra.

All three were focused almost exclusively on the Hoboken, N.J.-born saloon singer and master of the Great American Songbook whose music Mr. Mark never tired of, and with whom Mr. Mark had a decades-spanning friendship until Sinatra’s death in 1998.

In Sinatra-loving families, Mr. Mark’s radio shows — marked by his classy on-air presence and boundless enthusiasm for all phases of Ole Blue Eyes’ career, as well as his skill as a salesman for the show’s Italian food sponsors — connected generations.

Sundays With Sinatra’s promotional message summed it up: “Your grandfather listened. Your mother listened. And now you listen.” And Mr. Mark’s shows regularly featured a clip of the two friends in conversation: “God bless you, Frank Sinatra,” Mr. Mark was heard to say. “I love you too, Sidney,” the singer responded.

On Tuesday, Audacy Philadelphia marketing manager David Yadgaroff said in a statement that Mr. Mark was the host of “the longest running, single artist, syndicated radio program in America,” which most recently ran from 11 a.m. to 1 p.m. this past Sunday, although it was a taped archive edition, as it has been in the recent past during Mr. Mark’s health struggles.

Yadgaroff added: “We lost a man who speaks for a living, whose audience has been enraptured by every word — eloquent and articulate. Sid’s radio career spanned 65 years, the last 22 at Talk Radio 1210 WPHT, and 43 years of national syndication. He presented the music of Frank Sinatra from the standpoint of a friend as well as a fan. … Sid was the leading authority on Frank Sinatra and his entertainment legacy.”

David Dye, the Philadelphia rock DJ whose Dave’s World show airs on WXPN-FM (88.5) on Sundays and who has been a mainstay on Philadelphia radio since the early 1970s, called Mr. Mark “a true gentleman who found his niche and served Frank’s people. I loved his work.” Dye noted that Mr. Mark’s devotion to Sinatra was such that his production company was called Orange Productions, named after the singer’s favorite color.

Stephen Johnson, who worked with Mr. Mark recording the Sounds of Sinatra syndicated program at his Masters by Johnson audio facility in Narberth from 1983 to 2005, recalled many associates of Sinatra coming to the Main Line to guest on Mr. Mark’s show.

“We did a whole 13-week series with Tony Bennett. Angie Dickinson was on the show. Bobby Rydell was a regular.” What they all were drawn by, Johnson said, was Mr. Mark’s professionalism. “He was as classy as they come. And he could sell anything. But more than anything, he loved the music.”

Pierre Robert, the WMMR-FM (93.3) deejay who recently celebrated 40 consecutive years on the air, marveled that Mr. Mark could do it for 65 straight years (save for one show he missed in 1999 when he was recovering from open-heart surgery).

“There are very few legends like Sid,” said Robert, who called Sinatra “the original rock star.”

“First of all, it’s about the loyalty,” he said. “And it’s beyond being an icon. It’s this brilliant, lovely gentleman, this kind, very understated, not flashy guy who really was brilliant.

“And that brilliance was exhibited in the craft and the art of his radio work, where he could weave in any detail about Frank’s life and career. The movies, the songs. And not just the hits, but the B-sides, the C-sides, and the D-sides. He would play something and you’d say, ‘Wow, I never heard that before.’”

Mr. Mark was born Sidney Mark Fliegelman in Camden and graduated from Woodrow Wilson High School before serving in the Army from 1953 to 1955. He was assigned to Camp Polk in Louisiana, where he later said he witnessed racism and experienced anti-Semitism. He was also struck by what he called the “horrendous” heat. “There was an old slogan,” Mr. Mark said. “Write to the boys in Korea and pity the boys at Camp Polk.”

He was working as a jazz DJ on the graveyard shift on WHAT-FM in 1956 when a coworker didn’t show up, and he decided to play an hour of Sinatra. That started a tradition that would last 6½ decades, as he moved to several radio stations.

In the early years, however, he worked as an engineer for legendary Philadelphia deejays Georgie Woods and Doug “Jocko” Henderson and on his own shows, where he would play various jazz artists. And from 1965 to 1975, he had a TV show called The Mark of Jazz that ran on WPHL (Channel 17) and later on WHYY (Channel 12).

He was particularly proud of the role he played in supporting Nina Simone’s career when she was living in Philadelphia and performing in Atlantic City in the late 1950s.

“In her autobiography, she said the reason for her success was a white Jewish disc jockey, Sid Mark,” he told All That Philly Jazz in 2016. “Nina was something else. We had hours of discussions on the numerous radio and TV shows we did together. … When I discovered her, she was just playing piano at a little joint in Philly at 22nd and Chestnut. It was a bar, and she wasn’t singing, she was just playing piano.”

Mr. Mark first met Sinatra when the Chairman of the Board summoned him to Las Vegas. “I was ready,” he told The Inquirer in 1997. “My bags had been packed since ’55.”

Mr. Mark and his first wife, Loretta, flew to the desert and were invited to dinner with Sinatra and friends, including Quincy Jones, Milton Berle, and Jack Benny. “He introduced us to everyone at the table, saying, `This is the kid that does the show in Philadelphia.’ ... He was amazing,’’ Mr. Mark recalled.

Sounds of Sinatra was first syndicated nationally in 1979, and Mr. Mark was on WWDB-FM until the station switched to a rock-and-roll format and canceled the show. The Daily News started a petition drive to keep him on the air, and he moved in 2000 to WPHT, where he would remain.

“The only thing I can say is that Frank Sinatra has been the soundtrack to my life,” Mr. Mark once told The Inquirer. “There’s not a thing in my life that has happened that I could not put a Frank Sinatra song to.’’

“I still thoroughly enjoy it. I want to do this until I can’t do it anymore,” Mr. Mark said in 2015. “But like Sinatra said, walk out the front door, before they show you the back door.”

Mr. Mark’s son Brian Mark said on Tuesday night that he planned to do new intros and wrap around segments to previously taped Sundays With Sinatra and nationally syndicated Sounds of Sinatra shows that will include archived material and rarely aired interviews between Mr. Mark and Sinatra.

In addition to his son Brian, Mr. Mark is survived by his wife, Judy Mark; daughter Stacey Mark; and sons Andy and Eric Fliegelman. No funeral or memorial services are currently planned.