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.
Chief 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, Chief 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, Chief 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. Chief 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.
Chief Justice Theis’ honors include the American Constitution Society’s Legal Legend Award, the CBA’s Vanguard Award recognizing persons who have made the law and legal profession more accessible to and reflective of the community at large, the IJA’s Celebrating the Achievement of a Judicial Icon, the CBA’s John Paul Stevens Award, the Juvenile Justice Initiative’s Champion of Children’s Rights Award, the WBAI’s Ruth Bader Ginsburg Award Judicial Achievement Award, and the Illinois Bar Foundation’s Distinguished Service to Law and Society Award.
She is married, and has two children, and seven grandchildren.