Rendell was born Phyllis Mary Oliver to farmer John Oliver in the Falkland Islands and attended Darwin Boarding School before completing her education in Derbyshire, going on to study at Goldsmiths, University of London.[2] She qualified as a teacher in the UK and returned to the Falklands to work in Stanley as a teacher.[2]
In the 1970s, she met and married Mike Rendell, who was stationed in the Falklands as part of Naval Party 8901.[2] After working for six years in Saudi Arabia, Rendell returned to the Falklands in 1982 shortly after the end of the war.[3] In 1984, she became Director of the Camp Education Department in the Falkland Islands Government and then Director of Education in 1988.[2]