Mary Jane Theis (born February 27, 1949) is a justice of the Illinois Supreme Court for the First Judicial District in Cook County.

She was born Mary Jane Wendt in Chicago to Eleanore and Kenneth Wendt, a member of the Illinois General Assembly and a longtime judge in Cook County.

[6] In 2010, Theis was appointed by the Illinois Supreme Court to fill a vacancy on its bench, taking office October 26, 2010.

[8] In the general election, Chicago Tribune endorsed her for election to a full term, citing her "universally accepted expertise on judicial ethics, family law, sentencing, fairness.

"[9] She previously served as chair for the Illinois Judicial Conference's Committees on Education and Judicial Conduct, as well as on the Illinois Supreme Court's rules committee.