Among the more popular and notable projects he directed were the films A Dog of Flanders (1959), The Sad Horse (1959), Misty (1961), Flipper (1963), Island of the Blue Dolphins (1964), and My Side of the Mountain (1969), and episodes of the television series My Friend Flicka (1955–1956), Batman (1966–1967), and Lassie (1969–1971).
But in 1937 he moved to California and found work as a film editor at 20th Century Fox, and later married Isabel O'Brien.
In 1942, he edited Henry Hathaway's Oscar-nominated Ten Gentlemen from West Point, and the musical film Stormy Weather (which featured an all-African American cast) in 1943.
Throughout the 1940s and the 1950s, he was one of Hollywood's most reliable film editors, working on such high-profile projects as Nunnally Johnson's Oscar-nominated Holy Matrimony (1943), the religious epic The Keys of the Kingdom (1944), John M. Stahl's Oscar-winning Leave Her to Heaven (1945), Howard Hawks' comedy I Was a Male War Bride (1949), the 1951 military biopic The Desert Fox: The Story of Rommel, Sam Fuller's Cold War drama Hell and High Water (1954), and the highly popular Cary Grant-Deborah Kerr romance picture An Affair to Remember (1957).
His longest-running tenure as director on network television, however, occurred on the popular Batman series on ABC.
At the very end of his directorial career, Clark directed four episodes of the long-running television series Lassie, two each in 1969 and 1971.
They shared a Directors Guild of America Award for Outstanding Directing for a Television Film in 1967 for this effort.
His most popular and critically praised motion pictures focused on people's relationships with animals and the wild: A Dog of Flanders (1959), The Sad Horse (1959), Misty (1961), Flipper (1963), Island of the Blue Dolphins (1964), and My Side of the Mountain (1969).
A Dog of Flanders was widely praised for its performances and lush, painterly cinematography, while Flipper proved highly popular and led to a long-running television series (with which Clark was not associated).