Lyle Mays, a keyboardist, composer and arranger best known for his long association with the guitarist Pat Metheny, died on Monday in Simi Valley, Calif. He was 66.

His death, at Adventist Hospital, was announced by his niece Aubrey Johnson, a jazz singer. She did not specify the cause, saying only that he died “after a long battle with a recurring illness.”

Mr. Metheny has long been one of the marquee names in jazz. But the albums he made with his working band from 1978 to 2005 for ECM, Geffen and other labels were always credited to the Pat Metheny Group, never just to Mr. Metheny. And Mr. Mays was an integral part of that lean quartet from its early days, whether giving depth and color to its sound on synthesizers or soloing gracefully on grand piano.

The group gained fame by merging jazz ideas with a rock sensibility; its later incarnation as a larger ensemble incorporated musical ideas from other parts of the world, notably Brazil.