Cassel Hospital was founded and endowed by the philanthropist Ernest Cassel in 1919. Established as the ‘Cassel Hospital for Functional Nervous Disorders’, it was initially conceived that the hospital would focus on the treatment of shell-shocked soldiers. However, following a visit to Seale Hayne hospital in Devon by Ernest Cassel and the pioneer psychiatrist Sir Maurice Craig, it was decided that the new hospital would instead be open to all 'educated people of moderate means, who were suffering from functional nervous disorders'.
The house and accompanying grounds at Swaylands in Penshurst, Kent were purchased and converted into the hospital. Sir Ernest Cassel was joined by Sir Robert Hudson and Sir Courtaild Thomson on the Board of Trustees. Sir Maurice Craig, Dame Louis Aldrich-Blake, Sir Farquhar Buzzard, Lord Dawson of Penn, Sir Henry Head, Dr AF Hurst and Sir Frederick Treves served on the newly formed Medical Committee. Dr TA Ross was appointed as the first Medical Director and Physician.
Cassel Hospital opened in May 1921 providing accommodation for 54 patients. The Hospital proved a success, and in 1927 a separate Nurses' Home was constructed in the place of the stables and coachman's quarters to increase space for the growing number of patients within the house. By 1931 further alterations to the building allowed the Hospital to accommodate up to 64 patients.
In 1940 Swaylands was requisitioned by the army and the Hospital was evacuated to temporary premises at Ash Hall in Stoke-on-Trent. In 1947 the Hospital relocated again to Morgan House, Ham Common in Richmond which at the time was operating as a private business - Lawrence Hall Hotel.
From it’s opening in 1921 treatment at the Cassel was tailored to the specific needs of the individual with methods adopted from various schools of thought. This changed with the arrival of Thomas Forrest Main as Medical Director in 1946 and Doreen Weddell as Matron who together model the Hospital as a therapeutic community, practicing group psychotherapy and psychosocial nursing. For the first time the Hospital admitted mothers with babies, and began to practice family psychiatry.
Cassel became a leading centre for practice, training and experiment in the psychotherapeutic treatment of neuroses, training both nurses and doctors through formal course and a programme of talks.
In 1948 Cassel became part of the new National Health Service as a single unit hospital group under the South West Metropolitan Regional Hospital Board. Management of the hospital fell to the Hospital Management Committee. In the 1974 reorganisation of the NHS the Cassel was incorporated into the North West Thames Regional Health Authority and the South Hammersmith District Health Authority. Since 1982 the Cassel has been part of the Riverside Health Authority. From 1999 it was managed by the Earling, Hammersmith and Fulham Mental Health NHS Trust. The Cassel continues to have a national catchment area.