Mildred Leonora Sanderson (May 12, 1889 – October 10, 1914) was an American mathematician, best known for her mathematical theorem concerning modular invariants.^[1][2]

Sanderson was born in Waltham, Massachusetts, in 1889 and was the valedictorian of her class at the Waltham High School.^[1] She graduated from Mount Holyoke College in 1910, winning Senior Honors in Mathematics.^[1] She obtained her Ph.D degree from the University of Chicago in 1913,^[3] publishing the thesis (Sanderson 1913) in which she set forth her mathematical theorem. She was Leonard Eugene Dickson's first female doctoral student.^[1][3]

After completing her Ph.D., Sanderson briefly taught at the University of Wisconsin before her untimely death in 1914 due to tuberculosis.^[1][4]

Sanderson's theorem (Sanderson 1913, p.490) states: "To any modular invariant ${\displaystyle i}$ of a system of forms under any group ${\displaystyle G}$ of linear transformations with coefficients in the field ${\displaystyle {\rm {GF}}[p^{n}]}$, there corresponds a formal invariant ${\displaystyle I}$ under ${\displaystyle G}$ such that ${\displaystyle I=i}$ for all sets of values in the field of the coefficients of the system of forms." Often this theorem was cited as “Miss Sanderson’s Theorem”.^[1]

She is mentioned in the 2008 book Pioneering women in American mathematics: the pre-1940 PhD's, by Judy Green and Jeanne LaDuke.^[4]