Fortnum was born in Britain on 23 December 1919, at Harrow, Middlesex. She trained in Tunbridge Wells and then at the Central School of Arts and Crafts in London. She worked as an art teacher, painter, and textile designer before becoming a full-time book illustrator. She has illustrated nearly eighty books (2015). Her most well-known illustrations are of Michael Bond's character, Paddington Bear. The first illustrations of Paddington were made by Fortnum for "A Bear called Paddington".
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 niece, Caroline Nuttall-Smith. As of 2012, Fortnum resides in Essex, England.