Unlike his fellow Guild-founder, Ralph Morgan, who abandoned a law practice for acting, young Robertson did the reverse: he left professional acting for over six years to become a lawyer. In 1907, at 21 years of age, he landed a small role in David Belasco's huge Broadway hit The Warrens of Virginia, with future film director Cecil B. De Mille, starring future movie star Mary Pickford. Robertson remained in the theatre through 1912, then left to study law in Washington, D.C., passed the bar exam, and began practicing as a criminal lawyer for the government. The short-lived career proved invaluable training for his future screen appearances as lawyers, politicians, and various law enforcement agents. By 1920, he was back in the theatre, and would remain until 1930 as actor, playwright, and silent screenwriter – four of his plays were produced on Broadway over that decade, one of which he directed. The Pasadena Playhouse presented his play Desire in 1925. His Hollywood film career commenced in 1930 and he had appeared in supporting roles in three-dozen films by 1933, most noticeably as child star Jackie Cooper's father in the 1931 films Skippy and Sooky.