J.D. Souther, the singer and songwriter who wrote twangy but debonair hits for the Eagles and Linda Ronstadt that helped define the Southern California country-rock sound of the mid-1970s, has died. He was 78.

His death was confirmed by an Eagles spokesman, who said Souther died at his home in New Mexico without giving a cause or time of death. The musician was scheduled to begin a tour in Phoenix next week.

Souther — whose best-known songs included the Eagles’ “New Kid in Town” and “Heartache Tonight,” Ronstadt’s “Faithless Love” and his own “You’re Only Lonely,” which scored a Top 10 pop hit in 1979 — was also an actor, appearing in the TV series “Thirtysomething” and “Nashville” and in films such as “My Girl 2” and “Postcards From the Edge.” Other artists who recorded his songs included Bonnie Raitt, James Taylor, George Strait and the Dixie Chicks.

In January, Souther performed with the Eagles at the Kia Forum in Inglewood, where Don Henley introduced him as part of the “tight-knit community of songwriters and singers” that he and the Eagles’ Glenn Frey would turn to in the ’70s “when we got stuck on a song or we were trying to start new material.” He added that Souther was partly responsible for three of the Eagles’ five No. 1 singles, including “Best of My Love,” a tender, harmony-drenched ballad about a man “lying in bed, holding you close in my dreams / Thinking about all the things we said, and falling apart at the seams.”

John David Souther was born in Detroit but grew up in Amarillo, Texas, where he played jazz drums before taking up guitar. He moved to Los Angeles in the late ’60s and met Frey, with whom he formed the short-lived duo Longbranch Pennywhistle; the group built a following at the Troubadour in West Hollywood and released a debut album in 1969 before disbanding the following year.

Read more: A lap of honor in the making after 50 years: The Eagles say goodbye (maybe) in the Forum

Souther subsequently pursued a solo career while Frey took a gig backing up Ronstadt, with whom Souther was in a relationship; Henley joined Frey in Ronstadt’s band, along with guitarist Bernie Leadon and bassist Randy Meisner, laying the groundwork for the four to eventually form the Eagles. David Geffen, whose label Asylum released the Eagles’ first LP in 1972, “sort of” asked Souther to join the group, Souther told The Times in 2008.

“I thought about it and we rehearsed a set and played it one afternoon for David (and Eagles managers) Elliot Roberts and Ron Stone at the Troubadour,” Souther recalled. “It really only took me a minute to say, ‘No, the band was already exceptional,’ and I was just so happy to be able to stay home and write. I think they were relieved, too.”

In 1973, Souther formed the Souther-Hillman-Furay Band with Chris Hillman of the Byrds and Richie Furay of Buffalo Springfield, which released a pair of well-received country-rock albums. Souther resumed his solo work with 1976’s “Black Rose”, which featured a duet with Ronstadt on “If You Have Crying Eyes”, and 1979’s “You’re Only Lonely”, whose title track topped Billboard’s adult contemporary chart and peaked at No. 7 on the all-genre Hot 100.

After 1984’s “Home by Dawn” failed to match that commercial feat — the LP was “that unfortunate curiosity that was later called a ‘critical success,’” he said in a 1990 Times interview, “which means nobody bought it” — Souther took a break from recording, discouraged in part by the music industry’s growing reliance on MTV. “I wasn’t a big fan of music videos because I felt they encouraged a production overload rather than a real focus on the core of the music,” he told the New York Times in 2012.

Still, he scored a hit as a songwriter in 1989 with Henley’s MTV-approved “The Heart of the Matter,” which he wrote with the Eagles star and Mike Campbell of Tom Petty’s Heartbreakers. That same year, he appeared in his first film, playing a singer singing “Smoke Gets in Your Eyes” at a party in Steven Spielberg’s “Always.”

Souther, a two-time Grammy nominee and inductee into the Songwriters Hall of Fame, later moved to Nashville and returned to recording in 2008 with the jazzy “If the World Was You,” which he quickly followed with several more albums and a recurring role as a seasoned country music fixer on ABC’s soap opera “Nashville.”

Asked what inspired him to start recording again, he told The Times: “I probably stopped making records because I thought making records was making me crazy. Turns out I was already crazy.”

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This story originally appeared in the Los Angeles Times.