Mitchell studied with Muhal Richard Abrams and played in his Experimental Band, thus becoming one of the first members of the Association for the Advancement of Creative Musicians (AACM). The first AACM album to be released was Sound by Mitchell's sextet. Sound, in contrast to the predominate fast paced screaming sessions of the jazz avant-garde, was an exploration of silence and made great use of both traditionally non-jazz instruments but also the so-called "little instruments" or hand percussion.
He is the founder of the Creative Arts Collective, an organization similar in intent to the AACM.