Elliot Roberts, who managed the careers of musical stars Joni Mitchell, David Crosby, Jackson Browne, Tom Petty, Tracy Chapman and others but was best known for his five-decade association with Neil Young, died on Friday in Los Angeles. He was 76.

His company, Lookout Management, announced the death but did not state a cause.

Few pop artists have had business relationships as close and long-lasting as the one Mr. Young had with Mr. Roberts. After they met in a Los Angeles recording studio in 1967, Mr. Roberts became Mr. Young's advocate, adviser and comic foil.

In a remembrance posted on his website on Saturday, Mr. Young wrote: "When it came to our business, Elliot guided me through every move. We talked every day. Often I would call him multiple times a day, arguing, discussing, planning and sharing. He was there for me and protected my music with a fierceness."

Along with David Geffen, Mr. Roberts was one of the crucial figures behind the singer-songwriter scene in Southern California in the late 1960s and early '70s. The catalyst for both men's rise was Ms. Mitchell, whom Roberts saw perform at the Café Au Go Go in New York's Greenwich Village in 1967. She had a growing catalogue of innovative, powerful songs, but no recording deal.

"I went up to her after the show and said, 'I'm a young manager, and I'd kill to work with you,' " Mr. Roberts told Vanity Fair in 2015. He became her manager, and Mr. Geffen became her booking agent.

After Mr. Roberts secured Ms. Mitchell a deal with Reprise Records, the two relocated to Los Angeles, where in late 1967 she began recording her debut album, Song to a Seagull, at the Sunset Sound studio in Hollywood. The album's producer was David Crosby of the Byrds, whom Ms. Mitchell had recently met on tour in Florida. (Mr. Crosby also became client of Mr. Roberts.)

Also at Sunset Sound at the time of those sessions was Buffalo Springfield, Mr. Young's band, which was recording what would become its final album, Last Time Around. Ms. Mitchell had known Mr. Young from the Canadian folk scene and introduced him to her manager. Months later, after Buffalo Springfield disbanded, Mr. Young asked Mr. Roberts to manage his solo career.

Before long, Mr. Roberts and Mr. Geffen had emerged as kingpins of the new scene. They helped secure a record deal for a band that Mr. Crosby had formed with Stephen Stills and Graham Nash - a feat that included extricating Mr. Crosby and Mr. Nash from their existing contracts - and later added Mr. Young to the group.

In 1971, Mr. Roberts and Mr. Geffen founded Asylum Records, and many of the artists on the label's early roster, such as the Eagles and Mr. Browne, were managed by the company the two men had set up, Geffen-Roberts.

While Mr. Geffen became known as a shrewd if ruthless mogul-in-training, Mr. Roberts took part in the rock ′n' roll lifestyle of his clients. Mr. Crosby once described him in a Rolling Stone interview as "a cat who is like us."

Yet, within the business, Mr. Roberts also had a reputation for vigorous advocacy on behalf of his clients. That included taking advantage of the uncertainty that followed a corporate regime change at Warner Communications in the mid-1990s to secure a US$25-million deal for Mr. Young, according to Exploding: The Highs, Hits, Heroes, and Hustlers of the Warner Music Group, a 2002 book by Stan Cornyn, a former Warner Bros. executive.

"Elliot Roberts was probably the kindest, gentlest, and far and away the funniest man I ever worked with in show business," Mr. Stills said in a statement. "He was also tough as a barbed-wire fence, fiercely loyal and keenly observant."

Mr. Roberts was born Elliot Rabinowitz in the Bronx borough of New York on Feb. 25, 1943. By his early 20s he was climbing the lower rungs of the entertainment business. He worked in the mail room of the William Morris Agency in New York, where he met Mr. Geffen, then a young agent. He also managed a rock band, Robert's Rules of Order, around this time. He changed his legal name to Elliot Roberts in 1967.

He leaves his sons Jason, Ethan, Matthew and Zackary, and his partner, Dana Fineman.

Mr. Roberts's management partnership with Mr. Geffen had dissolved by the mid-1970s. Asylum had been sold to Warner Bros., and Mr. Geffen had sold his shares in the Geffen-Roberts management company to Mr. Roberts.

Mr. Roberts represented Ms. Mitchell until 1985, and at points in the 1970s and '80s, he worked with Tom Petty, Devo, the Cars, Yes, Ms. Chapman and Bob Dylan. More recently, he and his company, Lookout, represented Spiritualized, Mazzy Star and Devendra Banhart.

But Mr. Roberts never stopped working with Mr. Young.

"He was my best friend in the world for so many years," Mr. Young wrote in his tribute.