After graduating, Len Roberts-Smith went to Papua New Guinea in 1970, where he held various positions in the Crown Law Department, ultimately becoming Chief Crown Prosecutor. On Independence in 1975, he became the first public prosecutor under the new Constitution. As public prosecutor, he had responsibility for all criminal prosecutions throughout the country. He returned to South Australia in 1976 and became a stipendiary magistrate. In 1978, he was appointed the foundation director of the Legal Aid Commission of Western Australia and held this position for 11 years. In February 1989, he returned to private practice, as a barrister, and was appointed a Queen's Counsel in December 1989.