Armstrong's silver screen career began in 1927 when he appeared in Pathé's silent drama "The Main Event". He appeared in 127 films between 1927 and 1964; very prolific in the late 1920s and early 1930s, he made nine movies in 1928 alone. He is best known for his role as director Carl Denham in "King Kong". Months later, he starred as Carl Denham again in the sequel, "Son of Kong", released the same year. He resembled "King Kong" producer and adventurer Merian C. Cooper, and Cooper used him in several films as more or less a version of himself. "The Most Dangerous Game" was filmed at night on the same jungle sets as "King Kong", which was shot during the day, with Armstrong and Fay Wray simultaneously starring in both pictures. In 1937, Armstrong starred in "With Words and Music" (also referred to as "The Girl Said No"), released by Grand National Films Inc. He also worked throughout the 1930s and 1940s for many film studios. Prior to World War II, in 1940, Universal Pictures released "Enemy Agent", about countering a Nazi spy ring. In the film, Armstrong co-starred with Helen Vinson, Richard Cromwell and Jack La Rue. In 1942, he was reteamed with Cromwell in "Baby Face Morgan," a notable B movie for PRC (Producers Releasing Corporation). Later in that decade, Armstrong played another Carl Denham-like leading character role as "Max O'Hara" in 1949's "Mighty Joe Young." This film was another stop-motion animation giant gorilla fantasy, made by the same "King Kong" team of Merian C. Cooper and Ernest B. Schoedsack.