Casey Kasem, who counted down top-40 music hits on the syndicated radio show he hosted from the reign of the Eagles to the age of Beyoncé and who entertained millions of cartoon watchers as the voice of Scooby-Doo’s human sidekick, Shaggy, died June 15 at a hospital in Gig Harbor, Wash. He was 82.

Danny Deraney, a publicist for his daughter Kerri Kasem, said that the hospital did not give an immediate cause of death. Mr. Kasem had Lewy body dementia, a progressive brain disorder, and was in hospice care.

In recent months, Mr. Kasem’s degenerating condition spilled into public view because of a family rift over his care. Three children from his first marriage accused his wife of prohibiting visits to Mr. Kasem (pronounced CASE-um) and blocking their input on his treatment.

Amid the family battle, the former disc jockey had lost the ability to walk unaided or speak without struggle — a traumatic end for a man once described by People magazine as “5’8” of tightly coiled ambition.”

For nearly two generations, Mr. Kasem’s husky and energetic voice on “American Top 40” was sonic wallpaper in teenagers’ bedrooms and provided company on lonely car rides, in workplaces and waiting rooms, on call waiting and even on airplane audio systems.

It was anodyne — yet irresistible.

Only TV host Dick Clark of “American Bandstand” rivaled him in longevity as a messenger of pop music for mass audiences. Like Clark, Mr. Kasem exuded a middle-American earnestness, down to his neatly parted hair and the sweaters that might have been pulled from the wardrobe of a 1950s television dad.

His syndicated show, which he hosted from 1970 until Ryan Seacrest succeeded him in 2004, was carried by hundreds of stations nationally and heard in more than two dozen countries, often via Armed Forces Radio or Voice of America.

“American Top 40” was a four-hour block of songs from the Billboard pop charts, celebrity anecdotes and letters from listeners, all presented by Mr. Kasem with studied pacing and a keen sense of drama. “He carried on a national conversation to draw people in,” said Ron Simon, a curator for radio and television at the Paley Center for Media in New York.

Tuning in to the show was a communal experience. It has been said that, at Mr. Kasem’s peak in the 1970s and early 1980s, motorists who had reached their destinations on time nevertheless sat with their cars idling in driveways and parking spaces waiting to find out which song would be No. 1. Many appointments were missed on Mr. Kasem’s behalf.

In addition to his fame from “American Top 40” and its later top-20 spinoffs, Mr. Kasem could claim the distinction of being one of the first veejays. His syndicated music-video show, “America’s Top 10,” went on the air in 1980, predating MTV by a year, and lasted for much of the decade.

Mr. Kasem parlayed his commercial success into a lucrative second career doing voice-overs for thousands of commercials and cartoons; he made his voice sound reedier to portray Shaggy in “Scooby-Doo,” the long-running Saturday-morning cartoon about a dog sleuth.

Mr. Kasem, whose father was an immigrant Lebanese grocer in Detroit, became one of the country’s most prominent Arab Americans. He used his industry clout to draw attention to stereotyped portrayals of Arabs in movies and on television. He helped lead a successful effort to persuade Walt Disney executives to change song lyrics in the 1992 musical film “Aladdin” that called an Arab homeland “barbaric.”

He also advocated on behalf of a range of humanitarian and liberal causes, including Palestinian independence, the homeless and animal rights. He was a vegan who turned down commercial work for Kellogg’s because cereal is consumed with milk.

Mr. Kasem drew a sharp line between his activism and his broadcasting career. “American Top 40,” sometimes ridiculed for its vanilla sensibility, was ungossipy, unopinionated and unobjectionable. But that was the point. The host once said that if he ever gave the appearance of using “American Top 40” for purposes other than entertainment, “it would compromise any credibility I have.”

To Mr. Kasem, the people making pop music were young strivers after the American dream. This view was not the hippest way of presenting rock-and-rollers. But Mr. Kasem — who professed little interest in the actual music — saw himself as a teller of success stories in the mold of Horatio Alger.

He and his writing staff selected upbeat vignettes from the lives of featured performers and used them as teasers to hold listeners’ attention through commercial breaks: “Coming up — the most successful female singer of the rock-and-roll era, who once lived in an abandoned office building in New York and looked for discarded food in garbage cans.”

After the commercial, his audience learned that the singer was Madonna.

On other occasions, he informed listeners that the Edge, the guitarist for the Irish rock band U2, came from a family too poor to buy him a guitar — “so the talented teen built one!”

People magazine called the format “pop-schlock trivia,” but, for many listeners, such tidbits were valued kernels of information, however corny.

“I just felt it was my job to show that there is no easy way to success,” Mr. Kasem told the New York Times in 1990, “and that anyone who gets even just one Top 40 hit deserves their moment in the sun. I accentuate the positive and eliminate the negative. That is the timeless thing.”

Between songs, Mr. Kasem broadcast a segment he called “long-distance dedications.” While not to everyone’s taste, many of the letters he read on-air were heartfelt — messages from a teenager whose boyfriend died in a car crash; from a young daughter to her financially strapped father; from a pregnant woman to her drug-dealing boyfriend.

Mr. Kasem carefully guarded his listeners’ feelings by monitoring the show’s musical selections to maintain the proper tone. During one unaired 1985 rehearsal, Mr. Kasem went on a profane rant after playing an up-tempo Pointer Sisters song that led into a long-distance dedication for a dead dog named Snuggles.

Mr. Kasem’s jarring commentary became a YouTube sensation years later. More in keeping with his on-air persona was his trademark sign off: “Keep your feet on the ground, and keep reaching for the stars.”

The catchphrase was not just another showbiz exhortation. It seemed to encapsulate his own unlikely rise after a less than auspicious start in radio.

Kemal Kasem was born on April 27, 1932, in Detroit to parents of the Druse faith. His parents had a strong impulse toward assimilation that he said sometimes had negative consequences.

His father “didn’t care if he ever went back to the old country, he loved America so much,” he told People. “Growing up, I didn’t root for the Arabs in [the Foreign Legion ad­ven­ture film] ‘Beau Geste.’ I was an American. I didn’t question that the Arabs were the bad guys.”

While attending Detroit’s Wayne State University, Mr. Kasem acted on the radio in shows such as “The Lone Ranger” until he was called to Army service during the Korean War. While stationed in South Korea, he spun bebop jazz records for the Armed Forces Radio network under the moniker “Crazy Casey.”

As a civilian, he had a middling career on radio stations in Cleveland; Buffalo; Oakland, Calif.; and Los Angeles. He hosted the Los Angeles-based dance-party TV show “Shebang” that Clark produced in the mid-1960s. He appeared in low-budget biker and horror films such as “The Cycle Savages” and “The Incredible 2-Headed Transplant.”

Mr. Kasem’s professional breakthrough came in 1970, when he and his business partners devised a syndication arrangement offering hours of programming to stations nationwide. Instead of an upfront fee, Mr. Kasem and his associates received a share of the advertising revenue. The model, called “bartering,” was widely adopted.

Thus “American Top 40” was launched in an era when FM radio was killing off the pop “single” format in favor of longer play sets catering to specific tastes.

Mr. Kasem became one of the highest-paid people in radio broadcasting. In 1989, a compensation dispute led him to leave his longtime syndication home, ABC Radio, to join rival Westwood One radio network for a contract reputedly valued at $20 million over five years. Mr. Kasem had a competing countdown show, “Casey’s Top 40,” during those years, before returning to “American Top 40” on the little-known AMFM Radio Network in 1998. He retired from all his shows in 2009.

His first marriage, to Linda Myers, ended in divorce. In 1980, he married Jean Thompson, an actress who played Loretta Tortelli on the sitcom “Cheers.” Besides his wife, survivors include three children from his first marriage, Julie, Mike and Kerri; a daughter from his second marriage, Liberty; and two grandchildren.

Although the arrival of MTV and other splintering forces dimmed his cultural dominance, Mr. Kasem transcended ever-shifting trends and tastes in music. His top-40 show was, in a sense, forever current. But it was also forever old, with reruns of his broadcasts setting in amber the fickle tastes of any particular week in bygone years.

He was not certain that his radio work was how he would best be remembered. He had a long and profitable second career doing voice-overs for thousands of commercials and cartoons, most notably playing Shaggy, whose comic displays of cowardice helped define “Scooby-Doo.”

“They are going to be playing Shaggy and Scooby-Doo for eons and eons, and they’re going to forget Casey Kasem — unless they happen to step on his star on the Hollywood Walk of Fame,” he told the Times in 2004. “I’ll be one of those guys people say, ‘Who’s that?’ about. And someone else will say, ‘He’s just some guy who used to be on the radio.’ ”