Mary Fedden was born in Bristol where she attended the city's Badminton School. At the age of 16, she studied at the Slade School of Fine Arts, London from 1932 to 1936. At the Slade, Fedden was a pupil of the theatre designer, Vladimir Polunin. When she finished her studies, she taught, painted portraits and created stage designs for Sadler's Wells Theatre.

After the war was over, Fedden developed her own style of flower paintings and still lifes, reminiscent of artists such as Matisse and Braque.
Fedden's subjects are often executed in a bold, expressive style with vivid, contrasting colours, although her work of 2005-6 uses a narrower tonal range. Her still lifes are often placed in front of a landscape, as she enjoyed the contrasting of disparate, even quirky elements. When using watercolours she emphasised the rough texture of her favourite Indian papers.

Fedden exhibited in one-person shows throughout the UK every year from 1947 until her death in 2012. These included the Mansard Gallery in Heal's Department Store in 1947, Redfern Gallery, London from 1953, the New Grafton Gallery, London from the 1960s, the Hamet Gallery from 1970, the Arnolfini Gallery, Bristol, Bohun Gallery, Henley on Thames from 1984 and at the Beaux Arts Gallery, London in the 1990s. A major exhibition of her work was held at the Royal West of England Academy in 1996.