Kellin's coarse-featured face, tired eyes and flat, monotone voice suggested that he had lived hard and fast. He was most often cast as tough guys, both good and evil. He made his Broadway debut in 1949 in "At War with the Army" and repeated his role in the 1950 film version with Dean Martin and Jerry Lewis. He worked in some 50 plays and won an Obie Award for his work in "American Buffalo" and earned a Tony nomination in 1956 for his acting in the musical "Pipe Dream".