Mary Kay O'Brien (born June 4, 1965) is an American judge and politician who serves as a justice of the Illinois Supreme Court since 2022.

O'Brien received her bachelor's degree from Western Illinois University in 1986 and her J.D.

[5] O'Brien was appointed a judge of the Third District of the Illinois Appellate Court to fill the vacancy created by the retirement of Thomas J.

[7] O'Brien was elected to the third district seat on the Illinois Supreme Court in 2022.

[9] The Illinois Supreme Court appointed Linda Davenport to the vacancy created by O'Brien's election.