Gwyneth Johnstone was the daughter of Augustus John and niece of Gwen John, two of the most prominent British artists of the 20th Century. Her mother, the pianist Nora Brownsword, met Augustus John when she posed for him shortly before the First World War. Gwyneth was brought up between London and Norfolk by her mother.
She shared a London studio with her father.A talented artist herself, Gwyneth went on to study at the Slade School between 1933 and 1938, where she forged a lifelong friendship with Mary Fedden. She had a very close relationship with her mother and they often travelled together to Provence, where they stayed in a village near St Tropez. Gwyneth later bought a house in Spain in the 1970s. It was the scenery of the South of France and Spain which inspired her own colourful Mediterranean landscapes, for which she is remembered.