Retired as director of string chamber music at the Thornton School of Music, the University of Southern California. For 23 years (1957–81), he played first violin for the Lenox Quartet. Still active as a performer, in 2006 he toured Korea, Alaska (the Fairbanks Festival of the Arts), Brazil (XI International Cello Encounter) and France (Franco-Américaine Rencontre de Musique de la Chambre, at the Château de la Bretesche in Missillac).
After his time with the Lenox Quartet, Marsh was the first violinist for the Philadelphia, Berkshire, Sequoia, Southwest and Pacific quartets, as well as the Palo Verde and Picasso trios. He is a frequent recitalist on both the violin and viola. As concertmaster, Marsh has played with the Seattle Symphony, the Seattle Opera, the Pacific Northwest Ballet, the Colorado Festival Orchestra and the California Chamber Symphony. He has collaborated in concert with artists such as Leon Fleisher, Stanley Drucker and Eudice Shapiro.
He has had faculty appointments at Indiana University, Ithaca College and Western Washington University, as well as artist faculty positions at Aspen, Tanglewood, Ravinia, Spoleto, Fairbanks and other festivals. Additionally, Marsh has conducted the Western Washington University Symphony, has served on the music panels of NYSCA and the National Endowment for the Arts, and is a founder of Chamber Music America.