Muellner has become a respected local jazz pianist and composer. He teaches jazz theory and harmony privately on a limited basis. His style includes a focus on harmonic movement, improvisation and musical interplay. He freelances with many top musicians and often plays piano for private occasions. Jazz critic and writer Neil Tesser cites that Tommy is a "pianist and tunesmith of stellar magnitude."

Hammond organ jazz master Joey DeFrancesco stated, "Tommy swings, grooves, is soulful, and in total command of his instrument, plus I love him, he's one of my favorites!" Tommy has worked with many jazz musicians, including Ira Sullivan, Eddie Daniels, Marvin Stamm, Bobby Ojeda, John Fedchock, Warren Kime, Mark Colby, Richie Cole, Von Freeman, Ron Dewar, Michael S. Smith, Isaac Redd Holt (of the Ramsey Lewis Trio), Donny Osborne and Barrett Deems of Louis Armstrong fame. Tommy has worked with vocalists, including Arlene Bardelle, Hinda Hoffman, Spider Saloff and Catherine Whitney.