One of the three daughters of artist Tom MOSTYN, Nancy was born in Bushey, Hertfordshire, possibly at the time her father was studying under Hubert Herkomer. She studied at St John's Wood School of Art and Royal Academy Schools from 1912 to 1915 where she won silver and bronze medals. Her teachers included John Singer SARGENT, George CLAUSEN and Sir William ORPEN. In 1915 she won the British Institute Scholarship in painting. This award was won two years previously by Leonard John FULLER, whom she later married.

After her marriage and the birth of her son she found less time for painting, restricting herself to some commercial work. In 1938 she and her husband moved to St Ives and she helped him run the ST IVES SCHOOL of PAINTING. Her sister, Dorothy MOSTYN was also an artist, and exhibited at the RA in 1920 with the painting, Spring, as reported in the St Ives Times (14 May 1920).  At the end of the War she also served a while as STISA's Gallery Curator. By 1948 she was showing predominantly child portraits and still lifes. On her husband's death in 1973 she took over the running of the School of Painting, helped in her last years by Roy RAY, playing an important role in ensuring the survival of the School. She died in St Ives.