Margaret Clark was educated at Newnham College for women, Cambridge, during the years around 1900 and was later awarded the degree Master of Arts (MA) by the University of Cambridge. She married the banker A.B. Gillett.
From 1903 to 1948 she visited South Africa several times, collecting plants in the South-Western Cape and the Transvaal, particularly after 1924. Many of her collecting trips were made in the company of General J.C.Smuts*, statesman and amateur botanist, and in 1930 she accompanied Smuts and the British botanist John Hutchinson on an expedition to Lake Tanganyika. Some plant species were named after her, including Acacia gillettiae, by J. Burtt Davy*.