Lyle Mays, a keyboardist, composer and orchestrator who helped carve a new channel for contemporary jazz with The Pat Metheny Group, died on Monday in Los Angeles. He was 66.

His niece, jazz vocalist Aubrey Johnson, announced his death on social media, noting that he passed away “surrounded by loved ones, after a long battle with a recurring illness.”

Metheny, the guitarist, composer and NEA Jazz Master with whom Mays had his deepest musical bond, issued a statement remembering him as “one of the greatest musicians I have ever known.” As core members of The Pat Metheny Group, Mays and Metheny won 11 Grammys together, including one for their most recent album, The Way Up, in 2005.

Mays was a musician of clear, analytical temperament, but within the dimensions of his style — a personal amalgam of post-bop pianism, classical impressionism, Brazilian music, electronic music, rock ‘n’ roll and much else besides — there was always a core of emotional expression. The radiant, affirmative character of The Pat Metheny Group can only be understood as a byproduct of Mays’ distinctive chemistry with Metheny, his fellow Midwesterner, musical omnivore and tireless technophile.

The heartland sprawl in Mays’ piano playing, along with his expressive use of synthesizers, helped define the sound of the PMG, as it was also colloquially known. His instrumental voice was second only to Metheny’s in that setting; as designers of the music they were often effectively coequals. On many of the PMG’s albums, starting with a self-titled debut on ECM Records in 1978, the compositions were credited to “Pat Metheny and Lyle Mays,” in a formulation that echoed the collegial parity of “Lennon/McCartney” for The Beatles.

The PMG toured aggressively, keeping an itinerary more like a jam band than a jazz ensemble, and its audience steadily grew. By the time of its third album, Offramp, the band was also winning Grammy awards.

Because its style was pegged as fusion, despite Metheny’s objection to the term, the PMG encountered some resistance from the jazz establishment. In time, though, its sound left an indelible impression on the musical landscape — and not just in the more commercialized regions of contemporary jazz. There’s a pronounced Metheny-and-Mays influence on bands like Brian Blade Fellowship, and especially on its pianist and co-director, Jon Cowherd.

“Lyle was my first big piano influence,” Cowherd tells WBGO. “I heard First Circle for the first time at the age of around 15. I listened to that album countless times and was taken with Lyle’s sense of orchestration and use of emotion in his solos that would build so beautifully in the development sections of the groups extended compositions. He had enormous technique, a gorgeous touch and an astonishing sense of restraint while creating some of the most beautiful pastoral melodies.”

Lyle David Mays was born in Wausaukee, Wis. on Nov. 27, 1953, to musical parents: his father played guitar, and his mother played piano. He was playing the organ in church by his early teens.

Improvisation was always a part of his musical training, but it took on a larger significance during his studies at the University of North Texas, home to the oldest jazz studies program in the country. He earned high distinction as a member of the school’s elite One O’ Clock Lab Band, composing or arranging all of the music for an album, Lab ’75, that earned a Grammy nod.

After graduation, Mays spent the better part of a year on tour with the Woody Herman Orchestra. Then he met Metheny, whose star was unmistakably on the rise.

“Within a few minutes of hearing him I felt, ‘Wow, we should play together,’“ Metheny told composer-arranger Richard Niles. “That’s one of those things where you recognize a brother or a kindred spirit immediately.”

In the Niles conversation, gathered in The Pat Metheny Interviews: The Inner Workings of His Creativity Revealed (Hal Leonard, 2009), Metheny continues: “I can literally remember the first notes we played together. Whatever the core of the Pat Metheny Group sound is, and has been ever since then, was there — it was there in the first four bars! As much as people may think we’ve worked something out, a lot of it is just the way we play together.”

Mays had a career separate from the PMG, and while it received less attention, it covered a lot of ground. His self-titled debut album, released on Geffen in 1986, featured Billy Drewes on saxophones and Bill Frisell on guitar, among others; a 2000 album, for Warner Bros. was accurately titled Solo: Improvisations for Expanded Piano. He composed and performed the music for The Tale of Peter Rabbit, a 1988 audio version of the Beatrix Potter children’s story, read by Meryl Streep.

And a 1993 album called Fictionary featured a state-of-the-art acoustic post-bop trio with bassist Marc Johnson and drummer Jack DeJohnette. It’s an illuminating listen, for the openness in Mays’ playing, and a certain rhapsodic arc that calls Keith Jarrett to mind.

Still, it will undeniably be Mays’ collaboration with Metheny, actively spanning more than 35 years, that cements his musical legacy. Their work together brought them into the direct orbit of stars like Joni Mitchell and David Bowie, and on a series of marquee tours around the world — but its most emblematic document might also be its most intimate, a 1981 album called As Falls Wichita, So Falls Wichita Falls.

Generally understood as a duo album, despite contributions by percussionist and vocalist Naná Vasconcelos, As Falls Wichita is a work of billowing ambition and American optimism. It’s also a deeply personal statement: among its themes is “September 15,” a dedication to jazz pianist Bill Evans, a mutual hero for Mays and Metheny, who had died on that date the previous year.

Reviewing As Falls Wichita for The New York Times, Stephen Holden pronounced it “an evocative quasiclassical music of shimmering translucence.”

Holden added: “Although Pat Metheny receives first billing on the album, it is Lyle Mays, playing piano, synthesizer, organ, and autoharp, who dominates the record, with Mr. Metheny skillfully developing his ideas on the electric and acoustic guitars and the bass. With the producer Manfred Eicher, the pair have fashioned a winning combination of electronic innovation and neo-Romantic lyricism that could prove very influential.”