Sumner was born near Seattle, Washington, and spent part of her childhood on the Yakama Indian Reservation in the Pacific Northwest. From there, she earned a B.S. with Honors in geology from the California Institute of Technology (Caltech), then attended the Massachusetts Institute of Technology for her Ph.D. which she completed in 1995. Sumner then returned Caltech for postdoctoral research, supported by an O.K. Earl Postdoctoral Fellowship.