Frankie Darro was born on Saturday, December 22, 1917 in Chicago, Illinois as Frank Johnson, Jr. His parents, Frank Johnson, Sr. and his wife Ada were known as The Flying Johnsons, a flying circus act with the Sells Floto Circus; it was a profession that his father attempted to train him in, and he cured Frankie's fear of heights by having him walk on a length of wire, and then gradually raised the height of it until his son had mastered the trick.

As a child actor, he appeared in many silent adventure, western, and serial pictures of the 1920s, becoming a very prolific actor as an adolescent. His convincing delivery of dialogue and his obvious comfort before the cameras kept him steadily employed. His most important role during the 1930s was the lead actor in Wild Boys of the Road, Director william Wellman's indictment of aimless teens vagabonding across America during the Depression; he appeared in Mervyn LeRoy's Three on a Match in 1932, and was the principal character in the James Cagney feature The Mayor of Hell (1933). Darro remained popular in serials, and co-starred with Gene Autry in Autry's first starring role in the serial, The Phantom Empire.

In 1922, while the circus was in California, his parents divorced, and their circus act ended along with their marriage. The growing film industry, however, found a use for a small child who could do his own Stunts and the young Johnson, renamed "Frankie Darro", appeared in his first film at the age of six.

Darro's name grew in stature but he himself didn't: he stood only five feet, three inches, limiting his potential as a leading man. His wiry, athletic frame and relatively short stature often typecast him as jockeys; Darro played crooked riders in Charlie Chan at the Race Track and A Day at the Races. In 1938 Darro joined Monogram Pictures to star in a series of action melodramas. Darro's flair for comedy gradually increased the laugh content in these films, and by 1940 Mantan Moreland was hired to play his sidekick. The Frankie Darro series was so successful that Monogram used it as a haven for performers whose own series had been discontinued: Jackie Moran, Marcia Mae Jones, and Keye Luke joined Darro and Moreland in 1940, and Gale Storm would be added in 1941. Darro may be most familiar to modern audiences as the voice of the unlucky Lampwick in Disney's Pinocchio.

His recurring malaria symptoms caused him to increase his alcohol intake for pain management, and this affected his career. As film and TV roles became fewer, Darro opened his own tavern; he named it "Try Later", after the answer he most often received when he asked Central Casting for work. This proved unwise, however, given Darro's heavy drinking. By the mid-1950s, he had become too risky for producers to hire steadily, although he did continue to play small parts well into the 1960s.

Darro served in the US Navy Hospital Corps during World War II where he contracted malaria. Upon his return to civilian life, Monogram welcomed him back and cast the perennially youthful Darro in its "Teen Agers" campus comedies. When that series ended, the studio gave Darro four featured roles in its popular Bowery Boys comedies. He was an accomplished athlete and performed various Stunts for other actors in various films. Because of his size and fitness, he was cast in his most famous but anonymous big screen role: Darro was the actor/operator inside the now iconic 7-foot tall "Robby the Robot" walking screen prop that debuted in the classic MGM science fiction film Forbidden Planet (1956).

While visiting one of his ex-wives, step-daughter Christy and friends in Huntington Beach, California, Frankie Darro died of a heart attack on Christmas Day, 1976, three days after his 59th birthday. His remains were cremated and his ashes were scattered into the Pacific Ocean.

Later in his life, Darro appeared on television in The Red Skelton Show, Bat Masterson, Have Gun–Will Travel, The Untouchables, Alfred Hitchcock Presents, and Batman (episodes 9 and 10); he also did voice-over work for various projects.