Judge Mary F. Weir was appointed as an Associate Circuit Judge by Governor Jay Nixon on May 10, 2013. Prior to her appointment, she began working in 2001 as a contract attorney with Hope House, Inc., and provided legal representation to victims of domestic violence and their children. She also represented juveniles and their parents in cases heard before the Jackson County Family Court through her work as "of counsel" with the Raith Law Firm.

Judge Weir served as an Assistant Missouri Public Defender from 1998 until 2001. She was also a law clerk to the Honorable Christine Sill-Rogers of Division 31 from 1995 until 1998. She taught at the University of Missouri-Kansas City Law School as an Adjunct Professor for nineteen years. She also serves as volunteer faculty for the National Immigrant Women’s Advocacy Project. She is the current chair of the Missouri Supreme Court's Human Trafficking and Domestic Violence Commission. She is a member of the National Judicial Network, the Kansas City Metropolitan Bar Association, the Independence Coordinated Community Council, the Lee's Summit Coordinated Community Council, Lawyers Encouraging Academic Performance (LEAP), and the Association of Women Lawyers – Kansas City.

Judge Weir received a J.D. degree from Washburn University Law School in Topeka, Kansas in 1995. She obtained a B.A. degree in Political Science from UMKC in 1991. She is a parishioner at St. Francis Xavier Church in Kansas City.