Dendy was born in 1855 in
Bryn Celyn, Llangoed in
north Wales. She was the daughter of John Dendy, Unitarian minister, and his wife Sarah Beard (1831–1922), eldest daughter of
John Relly Beard. Her sister was the social reformer
Helen Bosanquet and her brother was the biologist
Arthur Dendy (1865–1925). She was home educated, completing her education with a year at
Bedford College, London. She started her work at
Collyhurst Recreation Rooms,
Manchester and with the Lancashire and Cheshire Women's Liberal Association and Suffrage Association. She was invited to sit on the Manchester School Board in February 1896. In this position she visited many schools in Manchester with a particular interest in the treatment of children with
mental deficiency. In November 1897 she lost her seat on the School Board in an election, but at the Board's request continued her work. In October 1898, at a Memorial Hall meeting addressed by the
Duchess of Sutherland the Lancashire and Cheshire Society for the Permanent Care of the Feeble Minded was established. In December 1898 Dendy read a paper before the
Statistical Society published as 'Feeble-minded Children'. Dendy was the driving force that established a colony for the "feeble-minded". Dendy believed in separate development to avoid crime and these people passing their problems on to their children. She joined the
Eugenics Education Society some time after 1900. She argued that it should be legally possible to confine children who were "feeble-minded" and the
Mental Deficiency Act 1913 and the
Elementary Education Act 1914 enabled this to happen. ==Works==