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Justice Mary Kay O'Brien

Justice on the Third District Appellate Court of Illinois

Mary Kay O’Brien is an experienced and highly qualified judge with deep roots in the community and a long history of public service.

 Justice O’Brien has served on the 3rd District Appellate Court for over 19 years. She has heard more than 4,500 cases and served on numerous committees.

 Justice O’Brien earned her law degree, worked for former Attorney Generals Neil Hartigan and Roland Burris, worked in the Grundy County State’s Attorney’s Office, and private practice covering family law, criminal law, real estate, banking, and municipal law.


Justice Mary Kay O'Brien attended Joliet Junior College, earned a Bachelor of Science Degree in Political Science/Public Administration from Western Illinois University, and a Juris Doctor from the University of Illinois College. of Law. A few months later, Justice O'Brien became a licensed attorney. 

Justice O'Brien would go on to practice law for many years prior to becoming a justice. O'Brien practiced law at:

  • Cortina, Mueller, and O'Brien / Law Offices of Frank Cortina • Coal City, IL • Associate, May 1994–July 1997; Partner, July 1997–December 1999 • Family Law, Aggravated Traffic, Real Estate, Criminal Law.

  • O'Brien and Smith, P.C. • Bradley, IL • Partner, July 2000–December 26, 2003 • Banking, Real Estate, Municipal Work.


May 1994 – December 2003

January 1997 – December 2003

December 2003 – Present


• Cortina, Mueller, and O'Brien / Law Offices of Frank Cortina

• O'Brien and Smith, P.C.

Family Law, Aggravated Traffic, Real Estate, Criminal Law, Banking, and Municipal Work


  • Judiciary II Committee on Criminal Law, Chair • January 1999–December  2003 • Presided over death penalty reform. Sponsored expungement reform, sought significant changes in evaluations for sex offenders and those charged with violence against animals

  • Legislative Audit Commission • January 1998–December 2003 • Reviewed and approved financial and compliance audits of all state agencies, boards and commissions receiving state funds. Initiated significant oversight of centralized child support collection crisis in 1999.

  • Served on the Agriculture, Transportation, Education, Energy & Environment, Child Support Enforcement and Labor Committees.

  • Illinois/Kentucky Low Level Nuclear Waste Compact Commission, Commissioner • January 1998–December 2003 • Responsible for providing support for storage and disposal of low-level nuclear waste to waste generators.


Appellate Court Judge

  • Board of Directors Illinois Lawyers Assistance Program,
    November 2014–Present (have served in every Executive Board position).

  • Illinois Courts Commission, Commissioner 2020–Present, Alternate Commissioner for several terms.

  • Illinois Workers Compensation Commission, Alternate
    Commissioner, January 2004–Present.

  • Supreme Court Legislative Committee, Vice-Chair January 2004–Present.

  • Supreme Court Appellate Committee on Access to Justice,
    February 2021–Present.

  • Supreme Court Education Conference, Educator,
    2020 Education Conference.

  • Third District Appellate Court Administrative Committee,
    April 2019–October 2019.

  • Supreme Court Rules Committee, January 2002–December 2011.


Justice Mary Kay O'Brien's campaigned for the Illinois Appellate Court, Third District in 2002.

The people of the Third District of Illinois had and continue to show overwhelming support for O'Brien. She has served on the Third District Appellate Court for 18 years.

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