Later Mantler formed a large orchestra with pianist/composer Carla Bley, whom he later married. After the Guild's demise, Mantler toured Europe with Bley and saxophonist Steve Lacy in the mid-'60s, then helped form the Jazz Composer's Orchestra Association (JCOA), a nonprofit foundation that would perform, record, and commission new works for jazz orchestra. Mantler played on Bley's A Genuine Tong Funeral with Gary Burton and orchestra in 1967, then issued a two-record set of his music for the JCOA in 1968, which featured many major players like Pharoah Sanders, Don Cherry, and Rudd. It won plaudits and praises from the international and national jazz press.

Mantler later conducted at the Electric Circus in New York and appeared on bassist Charlie Haden's 1969 Liberation Music Orchestra. He coordinated the recording of Bley's massive Escalator Over the Hill in 1970-1971, a three-record opus with more contributions from instrumental superstars. Mantler formed the New Music Distribution Service (NMDS) in 1972 as a separate part of the JCOA to oversee distribution for his and other JCOA artists' records. He and Bley co-formed Watt Works in 1973, a label designed for producing their own records.

Mantler built a recording studio near Woodstock in 1975, and he received grants for composition from the Creative Artists Program Service, the National Endowment for the Arts, and the Ford Foundation's recording-publishing program. Unfortunately, the JCOA and NMDS encountered financial problems and eventually went bankrupt, but Mantler has remained active in subsequent years, touring with Haden's Liberation Music Orchestra, performing and recording with the Carla Bley Band, and (into the 21st century) issuing numerous albums of his music on the ECM label.