Nina Marion Blackett was born in London on 1 February 1900. Her father Arthur Stuart Blackett worked on the London stock exchange and her mother Caroline Maynard was an enthusiastic artist. Her brother was Nobel physicist Patrick Blackett.
She studied psychology and physiology at University College London and graduated in 1924 with a first class degree. After graduation she started work as an industrial psychologist. In 1927 she was invited by Prof. Elton Mayo to study with him in the US and observe the development of his Hawthorne experiment. After 2 years she returned to England and was occupied in industrial psychology until the birth of her son in 1932. From then until 1939 she worked for the Girls Public Day School Trust as a psychologist, researching the emotional problems that could influence girls at school. This led to her book The Human Problem in Schools (1938).
At the age of 26, Milner had begun to record her thought streams in a diary in an attempt to discover a 'central purpose in life'. In 1934, under the pseudonym Joanna Field, Milner published A Life of One's Own (1934). This was the first of three books in which she shares the method and discoveries of her diary keeping. The discovery of her unconscious mind through this study was an immense surprise to Milner and it led her eventually to train as a psychoanalyst. In 1943, following analysis with Sylvia Payne, she qualified as an analyst in the British Society.
Also in 1943 she began an analysis of a psychotic patient which would last more than 20 years. Milner produced a uniquely detailed account of this analysis in her book The Hands of the Living God (1969). She was also a talented artist and her book On Not Being Able to Paint (1950) is an important work on creativity.
She wrote and published many other books and papers during her long career as a psychoanalyst and continued to see patients until her early 90s. She died on 29 May 1998.