She published studies on the intellectual and social development of children and promoted the nursery school movement.

Isaacs was born in 1885 in Turton, Lancashire, the daughter of William Fairhurst, a journalist and Methodist lay preacher, and his wife, Miriam Sutherland.

Isaacs also helped popularise the works of Klein, as well as the theories of Jean Piaget and Sigmund Freud.

Between 1929 and 1940, she was an 'agony aunt' under the pseudonym of Ursula Wise, replying to readers' problems in several child care journals, notably The Nursery World and Home and School.

[citation needed] Isaacs argued that it is important to develop children's skills to think clearly and exercise independent judgement.

"For Isaacs, play involves a perpetual form of experiment ... 'at any moment, a new line of inquiry or argument might flash out, a new step in understanding be taken'".

"What imaginative play does, in the first place is to create practical situations which may often then be pursued for their own sake, and this leads on to actual discovery or to verbal judgment and reasoning".

[citation needed] During the Controversial discussions of the British Psychoanalytical Society, Isaacs presented an influential position paper of 1943 setting out the Kleinian view of phantasy .

[10] Her statement has however been criticised as a kind of 'pan-instinctualism', over-simplifying the full range and scope of phantasy to a purely instinctual aim".

In 1922, she divorced Brierley and married Nathan Isaacs (1895–1966), a metals trader who collaborated with his wife in her later work.