Margaret Brackenbury Crook was born in Dymock, Gloucestershire, England on May 5, 1886 to Ellen Brackenbury Crook and Rev. L.G. Harris Crook. She attended St. Anne's School at Oxford, and received her BA with first class honors in 1913 from the University of London, since at that time Oxford did not grant degrees to women. Crook also earned a diploma with distinction in anthropology in 1914 from Oxon. In 1917, she received a certificate from Manchester College, Oxford, and was first in the college on completion of the three year course of study.

From 1916-17, she went to France to work for the Society of Friends War Victims Relief Committee in the Marne and the Meuse.