Agar was born in Buenos Aires to a Scottish father and an American mother. She went to school in England and studied at the Slade School of Fine Art, London. Rebelling against her privileged upbringing, she left her husband and in 1929 moved to Paris with her lover, the writer Joseph Bard. In the 1930s Agar was a leading British exponent of Surrealism, although she remained distinct from the political and theoretical aspects of the movement. Agar made collages, paintings, objects and photographs and was the only professional British female artist who exhibited at the 1936 International Surrealist Exhibition in London.