Born Robert Emmett Harron in New York City, New York, U.S., he was second oldest child of nine siblings in a poor, working-class Irish-Catholic family. He attended the Christian Brothers school in Greenwich Village and beginning at the age of thirteen found work as a messenger boy for American Biograph Studios to help support his family. Within a year of working for Biograph, Robert Harron and Christian Brothers friend and classmate James Smith were noticed by newly hired director D.W. Griffith who put both young boys under contract and the pair began appearing in bit parts for the studio. His first film was the now lost 1907 Biograph short Bobby's Kodak. Harron quickly became a favorite of Griffith and Griffith began to give the 14-year-old increasingly larger film roles.
The teenaged Robert Harron was often cast by Griffith in the role of the "sensitive" and "naïve" boy, who was overwhelmingly sympathetic and appealing to American film-goers in the very early years of American motion pictures and not far removed from Harron's real-life persona; Harron was often described as a quiet and soft-spoken youth. It was these traits that helped garner much public interest in the young actor, especially amongst young female fans. In 1912 alone, Robert Harron appeared in nearly forty films at Biograph.
Harron is probably best recalled for his roles in the three epic Griffith films: 1914's Judith of Bethulia, opposite Blanche Sweet, Mae Marsh, Henry B. Walthall and Dorothy and Lillian Gish, 1915's controversial all-star cast The Birth of a Nation, and 1916's colossal multi-scenario Intolerance opposite such popular stars of the era Lillian Gish, Mae Marsh, Wallace Reid, Harold Lockwood, Carol Dempster and Mildred Harris. One of Harron's most popular roles of the era came in 1919 when he starred opposite Lillian Gish in the Griffith directed romantic film True Heart Susie.
Robert Harron's film career continued to flourish throughout the 1910s and he was occasionally paired with leading actresses Mae Marsh and Lillian Gish with romantic plots, often in roles that cemented his "sensitive boy" image. Harron had, in fact, a burgeoning off-screen romantic relationship with Dorothy Gish. By 1920 however, it was reported that film director D. W. Griffith's interest in the young actor had waned and when Griffith hired film actor Richard Barthelmess to star in his 1920 film Way Down East, Harron was inconsolable. On September 5, 1920, while in New York City to attend the film premiere of Way Down East, Harron fatally shot himself in the left lung in his hotel room with a revolver that he had placed in a jacket.
Although the death was officially ruled accidental, many historians believe that Harron committed suicide. However, many of those closest to the actor vehemently refuted that Harron, a devout Catholic, would ever have considered suicide. While on his deathbed, Harron reportedly confided to his boyhood priest that his gunshot wound was the result of an accident.
Robert Harron was 27 years old at the time of his death and had never married. His film career spanned nearly fifteen years and Harron had appeared in over two hundred films.