Justice Mary Kay O'Brien was born in Kankakee, Illinois and raised on her family’s farm in Reddick, Illinois. She is a graduate of Western Illinois University and received her Juris Doctorate from the University of Illinois. Justice O’Brien began her legal career at the Grundy County State’s Attorney’s Office before going into private practice.

Justice O’Brien was elected to the Illinois House of Representatives in 1996 and served until December 2003. Justice O’Brien spent her legislative career chairing the Judiciary II Criminal Law Committee and advocating for victims of sexual assault and domestic violence. Her legislative victories include expanding the statute of limitations for criminal sexual assault of a minor and requiring psychological evaluations for individuals accused of torturing animals.

Justice O'Brien was appointed to the Appellate Court on December 26, 2003. She was subsequently elected to the Appellate Court in 2004 and retained in 2014. During her tenure on the Appellate Court, Justice O’Brien served as a Commissioner on the Illinois Courts Commission, an Alternate Commissioner on the Illinois Workers Compensation Commission, a member of the Appellate Court Administrative Committee, Vice-Chair of the Supreme Court Legislative Committee, the Appellate Court Access to Justice Committee and the Board of Directors of the Illinois Lawyers Assistance Program Courts Commission. Justice O’Brien has also been recognized with awards from numerous community and legal organizations including the Star Award from the Kankakee Society of Women in the Law, the Zonta Club International Woman of the Year Award (Kankakee Chapter) as well as being the recipient of the Outstanding Alumni Award from Western Illinois University.

Justice O’Brien was elected to the Supreme Court on November 8, 2022. Justice O’Brien lives within the Third Appellate District with her husband and her three sons.