In 1947, aged 32, Powell became matron at St George's Hospital. Her age reportedly "raised eyeborws" among the medical establishment, which deemed her too young for the position, but she proved her critics wrong and would remain there for more than two decades (22 years). Powell was part of the government committee that recommended removal of the title "matron" from the National Health Service in 1968.
The title has since been gradually reintroduced to the NHS lexicon. Muriel Powell was knighted in 1968 for her services to nursing, including her tenure as matron of St George's Hospital, London.