In July 1917, Ruth Darwin began volunteer work at an 'English Canteen' established in the Argonne region in eastern France. It was one of around fifty Cantine des Dames Anglaises [English Ladies Canteens] organised by the British Committee of the French Red Cross, where English women served refreshments and organised entertainments for French soldiers. Ruth volunteered at a 'Foyer de Cantonnement', a canteen with recreation rooms where men could sit and read, smoke, play games and write letters. She continued to volunteer in France until May 1919. Darwin served as an Honorary Commissioner to the Board of Control for Lunacy and Mental Deficiency, 1921–1930, and Senior Commissioner, 1931–1949. She carried out voluntary work in many branches of mental health work, including Secretary of the Cambridgeshire Voluntary Association for Mental Health, Chairman of the Mental Health Workers' Association, Chairman of the Federation of Associations of Mental Health Workers and a member of the Regional Hospital Board. She was appointed a CBE in 1938. In 1929, with money from the estate of her father, Horace Darwin, she founded the Darwin Trust to foster research into ‘mental defects, diseases or disorders’ and had some involvement in the affairs of the Littleton House Residential Special School (LHS), a school established by her father and others in 1920 in the Old Rectory at Girton, Cambridgeshire, to support children with a history of mental ill-health, trauma or learning difficulties.