Mary K. O'Brien was born on June 4, 1965, in Kankakee, Illinois and raised on the family farm in Reddick, Illinois. She is a graduate of Reddick High School and Western Illinois University, Macomb, Illinois. She received a J.D. from the University of Illinois in 1993. She was elected to the Illinois House of Representatives in 1996 and served until December 2003. Judge O'Brien was appointed to the Appellate Court on December 26, 2003. Judge O'Brien and her husband Todd Gereaux have two children, Mason and Michael.