Silent films
Coghlan began his acting career in 1920, appearing with Jack Dempsey in Daredevil Jack. In 1922 he co-starred with Brownie the Dog in a film called Rookies, and in 1923 he played a small role in the Pola Negri film The Spanish Dancer. He also appeared in early Hal Roach Our Gang films, including the 1923 short Giants vs. Yanks, in which the gang, after having a baseball game called off, gets stuck in an elegant home, which they destroy. In 1924 Coghlan was again cast opposite Jack Dempsey in Winning His Way.
One newspaper story described Coghlan's rise to fame this way: "When the boy was seven years old, his great mop of hair, freckled face, genial grin, and likable personality attracted the attention of several directors who urged his parents to permit him to engage in screen work. Mrs. Coghlan finally consented and one day he was cast for a 'bit' role in Goldwyn's Poverty of Riches, in which he played the son of Leatrice Joy."
By the mid 1920s, Coghlan had caught the eye of one of Hollywood's leading directors, Cecil B. DeMille. DeMille called Coghlan "the perfect example of a homeless waif" and signed the boy to a long-term contract in 1926. The Los Angeles Times reported on the event:
DeMille Signs Child ActorSo far, Cecil B. DeMille hasn't run much to giving picture contracts to youngsters. But yesterday Mr. DeMille signified what he thought of Junior Coghlan by placing him under a long-term contract. The boy has appeared in several DeMille pictures, including 'The Road to Yesterday.' The boy gives promise of being another Wesley Berry, with the same impish glance, the same freckles and the same cleverness. Of course, the office of freckled boy of the movies is a fixed institution, and now that Wes Berry has gone and got himself married he can't pretend to like playing marbles in the movies. Little Junior is to be cofeatured with Eleanor Faire and William Boyd in Rupert Julian's The Yankee Clipper.
In 1927, Coghlan appeared in the baseball comedy Slide, Kelly, Slide, playing an orphan who became a mascot and inspiration for an ace baseball pitcher, played by William Haines.
By 1928, Coghlan was such a well-known star that the Los Angeles Times reported on his schooling as well as his film projects. By age 11, Coghlan was asking to play grownup roles. A newspaper article at the time reported that Coghlan, "like every other young and red-blooded American, desires to arrive at manhood as soon as possible. Long trousers is what he wants, but the motion picture claims him and demands that he stay in knee breeches."
Coghlan's final film on his four-year DeMille-Pathe contract was 1929's military academy drama Square Shoulders. Conceived as a silent film, Square Shoulders was transformed into a "talkie" by the expedient of adding sound to the final reel. Only the silent version is known to survive.
A 1929 newspaper story on Coghlan noted that the twelve-year-old actor was "recognized by the motion picture public as the leading juvenile screen player in the world."