The daughter of James Clerk Maxwell Garnett, a barrister, and his wife Margaret Lucy Poulton, daughter of Sir Edward Bagnall Poulton, her parents lived in Gainsborough Gardens, Hampstead, and the young Garnett was educated at St Paul's Girls' School, Hammersmith, where she befriended Shiela Grant Duff.
[1] Joining the Labour Party, in 1934 Margaret Jay was recruited by Herbert Morrison as a candidate for the London County Council (LCC), and she was its youngest member when elected at a by-election in Hackney South in 1938, shortly before Labour gained control.
[1] She was the last survivor of the "Hampstead middle-class Labour grandes dames" whom Morrison had groomed to take over the LCC.
A son, Peter Jay, was a journalist, leading economist and a former British Ambassador to the United States.
[9] At one time her son-in-law, married to Helen, was Rupert Pennant-Rea, a former deputy Governor of the Bank of England.