Mary Jane Theis was born in Chicago. She received her bachelor's degree from Loyola University Chicago in 1971 and her law degree from the University of San Francisco, School of Law in 1974. From 1974-83, she was an Assistant Public Defender in Cook County.
Justice Theis has served at every level of the Judiciary in the State of Illinois. In 1983 she was appointed an Associate Judge in the Circuit Court of Cook County, where she served for five years. In 1988, Justice Theis was elected to the Circuit Court, where she was assigned to both the Criminal and Chancery Divisions until 1993, when she was appointed to the Appellate Court, First District. She was elected to the Appellate Court in 1994. When Chief Justice Thomas R. Fitzgerald retired in 2010, the Supreme Court appointed Justice Theis to fill his vacancy on the Court.
In her 17 years on the Appellate Court, Justice Theis served as a Presiding Judge. She was Committee Chair of both the Committee on Judicial Education and the Committee on Judicial Conduct of the Illinois Judicial Conference, and a member of the Supreme Court Rules Committee. Justice Theis was President of the Appellate Lawyers Association and the Illinois Judges Association, as well as President and founding member of the Illinois Judges Foundation. She has been a member of the Board of Governors of the Illinois State Bar Association and the Board of Managers of the Chicago Bar Association and is a member of the Women's Bar Association of Illinois.
Justice Theis has taught at numerous judicial education programs, as well as conferences and seminars for the ISBA and CBA, and at Loyola University School of Law, Northwestern University School of Law, and the John Marshall Law School. She has received various awards, including the Lifetime Achievement Award from the Illinois Judges Association, Catholic Lawyer of the Year from the Catholic Lawyers Guild, Celtic Lawyer of the Year from the Celtic Lawyers Society of Chicago, the Mary Heftel Hooten Award from the WBAI, and the Access to Justice Award from the ISBA. Justice Theis is the recipient of the American Constitution Society Legal Legend - Honor from the Chicago Lawyer Chapter.
She is married to John T. Theis, and they have two children, Jack (Uma Amuluru) and Claire (Joshua) Merok, and five grandchildren.