Many great scientists have made important contributations to many related fields. Gauss, Euler and Newton each made many contributions to both math and physic.
One of the great scientists of last century is Albert Einstein and I wonder, did he make any contributations to math?
Einstein gave a tremendous impetus to the development of differential geometry as a tool in doing general relativity. His contribution went far beyond introducing the index notation that currently bears his name! To cite a quick example, he deserves the credit jointly with David Hilbert for proving that the differential equations describing space-time ("the Einstein equation") is the Euler-Lagrange equation of a suitable variational functional; see Einstein-Hilbert action.