Tommy Kirk, who was a busy star in the Disney universe as a child and young man, appearing in “Old Yeller,” “The Shaggy Dog,” “Swiss Family Robinson” and other movies in the late 1950s and early ’60s, but whose career was derailed when his homosexuality became too widely known and when drugs and alcohol got the better of him, died on Tuesday at his home in Las Vegas. He was 79.

The Walt Disney Company announced his death in a statement, which did not give a cause.

Mr. Kirk got into show business by accident. An older brother was auditioning for a part in Eugene O’Neill’s “Ah, Wilderness!” at the Pasadena Playhouse in California and took Tommy, then 12, along; the brother didn’t get cast, but Tommy did (though not in the part his brother auditioned for).

“An agent came backstage and introduced himself and gave me his card and said, ‘Would you have your folks call me?’” Mr. Kirk recalled in an interview with the film magazine Scarlet Street in 1993. “My mom and dad did, and I ended up signed with a Beverly Hills agency.”

He started getting television roles almost immediately, many of them on the live one-hour dramas that were ubiquitous in TV’s early days. Then he was cast as Joe, the younger of the two crime-solving Hardy brothers (Tim Considine played the other, Frank), in two “Hardy Boys” serial adventures broadcast on “The Mickey Mouse Club” beginning in 1956.

The next year Disney signed him to a seven-year contract and cast him as the boy, Travis, who befriends the canine title character in the classic family movie “Old Yeller,” famed for its no-nonsense ending, in which Travis has to shoot his beloved pet because the dog was exposed to rabies.

“I really cried,” Mr. Kirk said of his work in that scene. “You can’t fake it; you’ve gotta cry. You can’t just make a face.”

The Library of Congress added “Old Yeller” to the National Film Registry in 2019.

Beverly Washburn, who played a neighbor girl in the film and is now its last surviving cast member, recalled Mr. Kirk as “a brilliant child actor.”

“He was so gifted, and he was adorable,” she said in a phone interview. They became lifelong friends.

The emotional ending to “Old Yeller” shocked many a young viewer, but, she said, Mr. Kirk had a particular view about the movie.

“Tommy’s philosophy on it was, it’s about love and loss,” she said. “Life throws us some curves sometimes. Not everything is a white picket fence.”

That was certainly the case for Mr. Kirk. He appeared in a number of other Disney films over the next seven years, many of them silly, successful comedies like “The Shaggy Dog” (1959), in which his character turned into a sheepdog, and “The Absent Minded Professor” (1961) and “Son of Flubber” (1963), which both starred Fred MacMurray as a man who invents a rubbery substance with fantastic properties.

Mr. Kirk was also in “Swiss Family Robinson” (1960), the “Old Yeller” sequel “Savage Sam” (1963) and a few other Disney efforts, but by the end of his contract he was trying the studio’s patience. One incident, he said, caused the company not to renew his contract.

“I was caught having sex with a boy at a public pool in Burbank,” he said, as quoted in a Liz Smith column in 1992. “We were both young, and the boy’s mother went to Walt.”

By then he was drinking heavily and using drugs.

“I was high all the time,” he told Scarlet Street. “It was a terrible period in my life. So I can understand the studio letting me go.”

It was a period when being identified as gay was particularly damaging to a career, and a 1964 marijuana arrest — Mr. Kirk was one of nine people charged when the police raided a Hollywood party — didn’t help his marketability.

He made a few more movies over the next decade, but they had titles like “The Ghost in the Invisible Bikini” (1966) and “Blood of Ghastly Horror” (1967). There was also “It’s Alive” (1969), which, in a 1990 interview with The Lexington Herald-Leader of Kentucky, Mr. Kirk described as “a monster movie so cheap that the monster wore a scuba suit and had Ping-Pong balls for eyes.”

By the mid-1970s he had given up acting. He quit drugs and found strength in Christianity, which he said helped him with “purging myself of resentment and bitterness.” He founded a carpet and upholstery cleaning business. He made a few more movies in the 1990s and early 2000s, but for the most part he limited his show business activities to autograph conventions.

In 2006 the Disney Company named him a Disney Legend, an honor recognizing extraordinary contributions to the company.

“I don’t blame Disney for firing me,” he told Scarlet Street. “I was on drugs, and I was fooling around in ways totally incompatible with a family-oriented studio. I’ve accepted it. I’ve accepted the fact that my career was ruined by my behavior and no one and nothing else.”

Mr. Kirk was born on Dec. 10, 1941, in Louisville, Ky. His parents moved the family to California in 1944 so that his father could work in an airplane factory during the World War II boom years.

Despite his troubles while under contract to Disney, Mr. Kirk had a fond memory of Walt Disney himself that he often related. Still a child actor, he ran into him at a Beverly Hills hotel.

“He was with Hedda Hopper, the legendary columnist,” Mr. Kirk told The Orlando Sentinel in 1991. “He put his arm around me, and he said ‘This is my good-luck piece here’ to Hedda Hopper. I never forgot that.”

Information on his survivors was not immediately available.

Ms. Washburn, his “Old Yeller” co-star, who lived near him in Las Vegas, would see him often. He would always bring a box of chocolates when he came to dinner, she said. And they worked autograph conventions together. She remembered that whenever someone in uniform — police, military or other — would come up to buy an autograph, Mr. Kirk would refuse their money.

“He’d always say, ‘They’re our heroes, and I’ll always give them a free picture,’” she said.

Ms. Washburn recalled one appearance with Mr. Kirk in which she mentioned that the two were friends, and a perplexed girl in the crowd piped up.

“He’s the one who shot Old Yeller,” the girl said. “How can you still be friends with him?”