Born in Devon to authors George and Ruth Manning-Sanders, Joan spent her early life in Sussex, France, Devon and Cornwall. She was educated by her governess, Miss Florence Bridge, who encouraged Joan and her younger brother David to visualize their history and Bible studies, and to put down their images on paper.

At the age of twelve she was asked by Bernard WALKE to do a series of watercolours of the New Testament for his church at St Hilary near Penzance, Cornwall. Realising her talent, her parents arranged for her to have a studio at Sennen Cove. In 1927 she exhibited two paintings in the Daily Express Young Artists' Exhibition at the RBA Galleries, not realising that the qualification for entry was age 18 and over. Her work was so good the organisers thought her age to be 18 and not the 13 as stated on the entry form.

In 1928, at sixteen years of age, she came to national prominence as the youngest person ever to have a painting hung at the RA, receiving universal praise. In 1929 Faber & Faber produced an illustrated book on her art, and a similar book was produced in New York the following year. She then went on to study at the Chelsea School of Art and continued to exhibit at the RA until 1935.

During WWII she became involved with STISA, and was raised from Associate to full Member in 1944.  Shortly after this she married, becoming Joan Floyd, and after the birth of her sons Christopher and John moved to Canada. On returning to the UK, she helped her mother, Ruth Manning-Sanders, with research for her many books of folk and fairy tales.  Her home was in Penzance, Cornwall, where she died.