In 1963, Warren was appointed Honorary Clinical Assistant in Pathology and Honorary Registrar in Haematology at Royal Adelaide Hospital. Subsequently, he lectured in pathology at Adelaide University, then took up the position of Clinical Pathology Registrar at the Royal Melbourne Hospital. In 1967, Warren was elected to the Royal College of Pathologists of Australasia and became a senior pathologist at the Royal Perth Hospital where he spent the majority of his career.