Forster began his career as a house surgeon at the Birmingham Accident Hospital, in the West Midlands of England, in 1943. In 1944, he joined Warlingham Park Mental Hospital as a house physician.[2] That same year, he was appointed assistant medical officer at the Central Mental Hospital in Hatton.[2] He worked there for about two years, after which he was made a general practitioner in Birmingham, England. Forster returned to Gambia in 1946 and served under the colonial medical services until 1951.[7][8] He joined the Ministry of Health in Accra,[9] Ghana where he worked as a consultant in Psychiatry and later, doctor in charge of the Accra Mental Hospital.[10][11][12] He served in this capacity until 1970 when he gained employment at the University of Ghana.[2]
After serving for about 19 years as the doctor in charge of the Accra Mental Hospital, Forster was appointed associate professor of Psychiatry at the University of Ghana Medical School. In 1972, he was elevated to the status of a professor and head of the Psychiatry Department of the University of Ghana Medical School.[2][6][12][13] He was president of the West African College of Physicians from 1983 to 1984.
Forster was the son of Hannah Forster; the first Gambian female politician, and the brother of Catherine Collier; the first Gambian Radiographer.[14] He married Essi Matilda Forster (née Christian, who was the first female Gold Coast native to become a lawyer) on 17 December 1944.[12] Together, they had three children: one daughter and two sons.[2] Forster's hobbies were walking, and reading.[2]