She was born in British Guiana, and grew up in Georgetown. She and her fiancé Joe Phoenix moved to England in 1956, and married in June of that year.
Sybil Phoenix started fostering for Lewisham in 1961. She made her own hostel for girls and became a community worker. She has been involved in Black History Month and was given an MBE in 1973. In 1979 she founded the Marsha Phoenix Memorial Trust, a supported housing project for single homeless young women aged from 16 to 25, which is named in honour of her daughter who died in a car accident in 1974.
In 1987, Princess Margaret asked her to be Minister without Portfolio for Guyana.