Sumner grew up between Ollerset, her childhood home in Booysens, Johannesburg, and Eathorpe, her father’s ancestral family estate in Warwickshire, England. She completed her schooling at Roedean in Johannesburg and, although she knew from an early age that she wanted to become an artist, she first completed a degree in English literature at Oxford University at her father’s insistence. Sumner spent a year at the Westminster School of Art in London, but finding the English art scene moribund, moved to Paris in 1926. She attended the Ateliers d’Art Sacré under studio masters George Desvallières and Maurice Denis and, from 1934, the Académie de la Grande Chaumière. She returned to South Africa regularly in the interim, sometimes for a year at a time, to visit her family, paint, and exhibit her work, a pattern she repeated for most of her life, until ill health forced her to remain in Johannesburg from 1979. She first exhibited work at the Salon d’Automne in Paris in 1930 and 1931, and held her first solo show in Cape Town in 1932. She joined the New Group at Walter Battiss’s invitation in 1938.

Sumner’s early work reflects her interest in the work of Pierre Bonnard and Édouard Vuillard, as well as other avant garde French artists of the École de Paris. She spent the war years in Johannesburg, and on returning to her studio in Paris in 1947, noticed a growing emphasis on abstraction and heightened colour in the city's art scene. Her work took a turn towards greater fragmentation, influenced by the work of her friend, the artist Paul Berçot, and the Rayonnist painters. The first of many visits to Namibia in 1965 heralded another shift in her work, and the ‘luminosity of space’ and ‘silence’ became dominant themes, with forms dissolving into light and colour.

Sumner was a devout Catholic and executed a number of designs for stained glass church windows and paintings on religious themes, including fourteen Stations of the Cross for St Mary’s cathedral in Cape Town in the 1960s.