Mantler was born in Vienna, Austria. In the early 1960s, he was a student at the Academy of Music and Vienna University, concentrating on trumpet and musicology. He continued his studies at the Berklee College of Music in Boston. In 1964, he moved to New York City and joined the Jazz Composers Guild with Roswell Rudd, Archie Shepp, and Cecil Taylor. He founded the Jazz Realities quintet with Carla Bley and toured in Europe with Steve Lacy. After the Guild broke up, he established the Jazz Composer's Orchestra Association (JCOA). Its purpose was to provide an outlet for new orchestral jazz compositions. For its first record release he produced a double album of his music during 1968, entitled The Jazz Composer's Orchestra, with soloists Cecil Taylor, Don Cherry, Roswell Rudd, Pharoah Sanders, Larry Coryell, and Gato Barbieri. Some of this music was also performed during the "Long Concerts" at the Electric Circus in New York in 1969.
The problems of independently distributing the orchestra's record label led him to form the New Music Distribution Service (as a division of JCOA) in 1972, an organization which was to serve many independent labels for almost twenty years. Mantler and Bley started their own company, WATT, which was a record label, recording studio, and music publisher. By the mid 1970s both orchestra and distributor discontinued their activities.
Mantler recorded many solo albums with varying instrumentation and personnel, emphasizing his work as a composer rather than as a band leader. Appearing live infrequently, he concentrated on composing and recording. He recorded Something There with the string section of the London Symphony Orchestra and several albums using the words of writers Samuel Beckett (No Answer), Harold Pinter (Silence), and Edward Gorey (The Hapless Child).
Commissions and performances with European orchestras followed, at the Danish Radio, the Swedish Radio, the North and West German Radio, and the Lille Opera. His 1987 album Many Have No Speech contained songs in English, German, and French that were based on the poetry of Samuel Beckett, Ernst Meister, and Philippe Soupault, with vocals by rock musicians Jack Bruce, Marianne Faithfull, and Robert Wyatt.
From 1977 until 1985 he was also a member of the Carla Bley Band, touring extensively throughout Europe, the USA, and Japan, as well as appearing on all of the Band's recordings. Mantler and Bley were married from 1965 –1991 and had one daughter, Karen Mantler, who became a musician.
In 1991 he left the United States and moved to Europe, dividing his time between Copenhagen, Denmark and the South of France.
A new orchestral piece was commissioned by the Austrian Donau Festival, and was premièred near Vienna in June 1991 by the Nö.Tonkünstlerorchester, conducted by Michael Gibbs, with Andy Sheppard as soloist. New compositions were also commissioned by the Danish Radio Big Band and the North German Radio Big Band in Hamburg.