His parents were Percy F. Marshall and Ethel May Turner. He graduated from St. Mary's College in Old Harlow, Essex and worked for a time as an accounting clerk. Marshall overcame the loss of a leg in World War I, where he served in the London Scottish Regiment with fellow actors Basil Rathbone, Ronald Colman and Claude Rains, to enjoy a long career.
He was featured in both the 1929 and the more famous 1940 version of The Letter, first as the murdered lover, then the wronged husband.
He also starred in a popular radio series, The Man Called X, in which he played a globe-trotting "American" spy with an English accent.
He was married five times. Among his wives were two actresses, Edna Best, with whom he appeared in The Calendar, Michael and Mary and The Faithful Heart, and Boots Mallory, to whom he was married from 1947 until her death in 1958. His grave is located at Chapel of the Pines Crematory in Los Angeles.
Marshall had a daughter, Sarah, by Edna (his second wife) and two other, younger children.