[1][2] Costa was born Margaret Mary Murphy on 30 August 1917 in Umtali, where her father worked for the then Southern Rhodesian government.
[1][2] She moved to London during World War II to be a senior civil servant in the Ministry of Fuel and Power after a period working for Chatham House whilst it was in Oxford.
She became friends with Raymond Postgate and assisted him in compiling early editions of the Good Food Guide.
[2] In 1965, Costa took over the regular cookery column in The Sunday Times colour magazine from Robert Carrier.
She also wrote about food and travel for the American magazine Gourmet, where she promoted Albert and Michel Roux early in their careers.