Keen made his full film debut in 1946 in "Riders of the New Forest" but soon appeared in better known films for Reed such as: "Odd Man Out" (1947), "The Fallen Idol" (1948), and "The Third Man" (1949). He quickly became one of the busiest character actors, typically doing five films a year. He also continued to perform on stage, for instance as Iachimo in Peter Hall's 1957 production of "Cymbeline", and a sadistic Turkish General in Terence Rattigan's controversial play "Ross" (1960).