Born in Warwick, Warwickshire, England, and educated at King's High School for Girls there, Ansell qualified at Birmingham in 1946 and did her post-graduate training at Hammersmith. In 1951 she was appointed as registrar to Professor Eric Bywaters at the Canadian Red Cross Memorial Hospital, Taplow, Buckinghamshire, where she did research on heart disease in rheumatic fever.
In 1962, she was appointed clinical physician in rheumatology at Taplow. Appointed head of Division of Rheumatology at the Clinical Research Centre at Northwick Park Hospital in 1976. Awarded a scholarship to study in Chicago at the Research and Education Hospital as a research fellow. Awarded the CBE in 1982. Recognised with a Visiting Professorship at Leeds in 1997.
Ansell was based at the Canadian Red Cross Memorial Hospital, specializing in the research and treatment of Juvenile Idiopathic Arthritis. She developed a classification system for childhood arthritis. While focusing on treatment of the disease, she recognised the importance of maintaining educational and social skills in young patients.
She pioneered a team system of professionals including physiotherapists, occupational therapists, nurses, teachers, social workers, ophthalmologists, orthopaedic surgeons, dentists, and podiatrists in order to treat and manage her patients.
"During her life she made a major contribution to the understanding of children with Juvenile Idiopathic Arthritis and in developing services to treat them in the United Kingdom. Her influence was not restricted to this country, and by the time she retired from the Health Service in 1988, she was the world leader in the care of childhood arthritis." 
Ansell was author of over 360 papers in adult and pediatric rheumatology and was an honorary member or fellow of over 16 national and international societies.
After her death, a new science building at the Kings High School for Girls, and a street in Warwick (Ansell Way), were named in her honour.
- Clinics in Rheumatic Diseases (W. B. Saunders, 1976 London)
- Chronic Ailments in Childhood (1976)
- Rheumatic Disorders in Childhood (Postgraduate Paediatrics) (Butterworth-Heinemann Ltd), ISBN 0-407-00186-7
- Color Atlas of Pediatric Rheumatology (Mosby, 1991), ISBN 0-7234-1658-3