Susan Isaacs was perhaps the most influential English-born child psychologist of her generation. She published important studies of children's intellectual and social development; founded an experimental school–the Malting House School in Cambridge – and set up a Department of Child Development at the London Institute of Education; acted as a tireless propagandist for the nursery school movement; and, in general, presented a difficult subject intelligently and attractively to the general public, lecturing to schoolteachers, writing a weekly problem page in Nursery World, and giving evidence to the Consultative Committee of the Board of Education. Above all, she did more than anyone else of her generation to introduce educational psychologists to the work of Sigmund Freud, Jean Piaget and Melanie Klein.

Her main interest lay in ‘the psychology of child development’. She tried to understand children's intellectual and social development, recreating their conceptions of the world and untangling their most intimate social relations. She was one of the earliest exponents of Piaget's work in English, and initiated criticisms of his analysis of stages of cognitive development which have since become standard. Early in her career, she recognised the fundamental contribution of psychoanalytical theory to understanding human development, and she repeatedly emphasised the emotional needs and unconscious mental processes underlying even the most unremarkable behaviour. She was successively a practising psychoanalyst and a training analyst, and she played a leading role in popularising Melanie Klein's arguments.