J.D. Souther, the singer and songwriter who co-wrote twangy yet 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 a representative for the Eagles, who said Souther died at his home in New Mexico without specifying a cause or saying when he died. The musician was due to launch a tour next week in Phoenix.

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 gave him a top 10 pop hit in 1979 — was also an actor with roles on TV’s “Thirtysomething” and “Nashville” and in movies including “My Girl 2” and “Postcards From the Edge.” Among the other acts who recorded his songs were Bonnie Raitt, James Taylor, George Strait and the Dixie Chicks.

In January, Souther performed onstage with the Eagles at Inglewood’s Kia Forum, where Don Henley introduced him as part of the “tightknit community of songwriters and singers” that he and the Eagles’ Glenn Frey would turn to in the ’70s “when we would get stuck on a song or we’d try to start some new material.” He added that Souther was partially responsible for three of the Eagles’ five No. 1 singles, which also included “Best of My Love,” a tender, harmony-drenched ballad about a guy “lying in bed, holding you close in my dreams / Thinking about all the things that we said, and coming 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 the 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 West Hollywood’s Troubadour and released a debut album in 1969 before breaking up the following year.

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

“I considered it, and we rehearsed a set and played it for David [and Eagles managers] Elliot Roberts and Ron Stone at the Troubadour one afternoon,” Souther recalled. “Truthfully, it took all of a minute afterward to say, no, the band was exceptional as it was, and I was quite happy to stay home and write. I think they were relieved, as well.”

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

James Taylor, left, and J.D. Souther perform in Atlanta in 1981.

(Rick Diamond / Getty Images)

After 1984’s “Home by Dawn” failed to match that commercial performance — the LP was “that unfortunate curiosity that’s later called a ‘critical success,’” he said in a Times interview in 1990, “meaning 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 huge fan of music videos because I thought they encouraged an excess of production as opposed to a real focus on the heart of the music,” he told the New York Times in 2012.

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

A two-time Grammy nominee and a member of the Songwriters Hall of Fame, Souther — whose survivors include two sisters and a former wife and her daughter — later moved to Nashville and returned to record-making in 2008 with the jazzy “If the World Was You,” which he quickly followed with several more albums and a recurring part as a grizzled country music fixer on ABC’s soapy “Nashville.”

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