His family moved to Washington, D.C., in 1944, and in the fall of that year, he started commuting to New York City once a week for lessons with Daniel Bonade, former first clarinetist of the Cleveland and Philadelphia Orchestras.
He remained in this position until he was appointed principal clarinet in the Cleveland Orchestra at the invitation of George Szell in 1953.
Marcellus made his debut as soloist with the Cleveland Orchestra on March 29–31, 1956, when he played Wolfgang Amadeus Mozart's Clarinet Concerto in A major, K. 622.
In the summer of 1961, he played Mozart's Clarinet Concerto at the Casals Festival in Puerto Rico.
At his death he was a beloved and universally respected and admired artist and pedagogue whose conservative and highly disciplined approach to instrumental technique influenced generations of clarinetists.