Born Elizabeth Leah Perrett on 14 April 1886. When she was 14 her parents migrated to Canada but she decided to remain in the UK with her grandparents. She attended Homerton College, Cambridge and was working as a teacher in Cambridge when she met Hugh Dalton and joined the Fabian Society and the Independent Labour Party. She campaigned for free milk for the school children to improve their health.
In 1914 she married the astronomer William Henry Manning.
Manning joined the 1917 club after the Russian Revolution and became a speaker at Labour events around the country. She became headmistress of an Open Air School for under-nourished children in Cambridge. In 1929 she was organising secretary of the National Union of Teachers and became President the following year.
In February 1931 she was elected as MP for Islington East, only to lose her seat in the General Election when she decided not to support Ramsay Macdonald's National Government. She spent a year on the National Executive Committee of the Labour Party and stood as a candidate for Sunderland at the 1935 General Election but was unsuccessful.
She also began to move away from her previous stance as a pacifist to that of actively campaigning against fascism. At the 1936 Labour party conference she argued, along with Ellen Wilkinson and Aneurin Bevan, for military assistance to be given to the Popular Front of Spain in its fight against Franco. The appeal was unsuccessful and Labour supported the Conservative policy of non-intervention. Manning became Secretary of the Spanish Medical Aid Committee and was involved in organising the evacuation of 4000 children and 200 adults on the SS Habana which brought the refugees to Britain. Whilst in Spain she witnessed the bombing of the Basque town of Guernica on 26 April 1937.
In 1945 manning was elected as MP for Epping and developed a reputation for her commitment to education. She lost her seat in the 1950 election and was unsuccessful in 1951 and 1955.
She was awarded the title of Dame Commander of the Order of the British Empire in 1966 and remained active in the field of education including opposing the creation of comprehensive schools. Her autobiography A Life for Education was published in 1970.
She died on 15 Sep 1977. She was the subject of a posthumous biography in 1991 by Ron Bill and Stan Newens who were involved in the Leah Manning Trust.