Fortnum was born in Britain on 23 December 1919, at Harrow, Middlesex. She worked as an art teacher, painter, and textile designer before becoming a full-time book illustrator. She has illustrated nearly sixty-five books to date. Her most well-known illustrations are of Michael Bond's character, Paddington Bear.
Although her pen-and-ink drawings of Paddington were done in black and white, some of her work has been coloured by other artists, including her step granddaughter, Caroline Nuttall-Smith. As of 2012, Fortnum resides in Essex, England.