Margaret Christian (Peggy) Garnett was born in Manchester, 28 January 1913, the daughter of James Clerk Maxwell Garnett and Margaret Lucy Garnett (née Poulton). She was educated at Malvern Girls' College; St Paul's Girls' School; and Somerville College, Oxford. She married Douglas Jay in 1933 (divorced 1972), with whom she had two sons and two daughters.

She joined the Labour Party at Oxford and was elected later to the London County Council (LCC) as Labour member for Central Hackney, 1938-49, and then for North Battersea, 1952-67. During her time as a councillor of the LCC, she was chairman of the Schools' Committee; vice-chairman of the Welfare Committee; and a member of the Children's Committee. She served on the LCC's successor body, the Greater London Council (GLC), 1964-7, most notably as chairman of the Parks' Committee and as an instigator of "One O'Clock Clubs" for young mothers. In the early 1980s, she left the Labour Party to join the newly formed Social Democratic Party (SDP), eventually returning to the Labour Party in 2007.

She was also a member of the Royal Commission on Population; chairman of the Friern Psychiatric Hospital Management Committee; and the head of a committee of inquiry into the training of staff working with disabled patients.

In addition to her official responsibilities, she devoted much time to activism and voluntary work, particularly in Hampstead, where she lived almost all her life. She was chairman (later life president) of the Heath and Old Hampstead Society, 1968-89, and one of the campaigners for the preservation of Burgh House as a community arts centre.

She died in Hampstead on 21 January 2008.

Her publications include: "Better Schools Now!" (1953) and "Loves and Labours" (1990).