Commissioner Erin M. O'Connell-Diaz received her undergraduate degree in History and Government with honors from St. Mary's College of Notre Dame, South Bend, Indiana. She graduated from Loyola University School of Law in Chicago with a Juris Doctorate.

She was then appointed by the Illinois Attorney General to serve as an Assistant Attorney General in the General Law Division. In that capacity she represented over 150 state agencies in matters ranging from complex construction contract litigation to prisoner civil rights cases.

In 1991, she was appointed as an Administrative Law Judge for the Illinois Commerce Commission. In that role, Commissioner O'Connell-Diaz handled a full spectrum of dockets involving all aspects of utility regulation including electric, gas and water utility rate cases, consumer complaint cases, telecommunication arbitration and certification cases. Commissioner O'Connell-Diaz also served as the Administrative Law Judge on Commonwealth Edison Company's franchise fee case and both its Delivery Service Tariff cases. In 2001, she was appointed Assistant Director of the Administrative Law Division. Additionally, Commissioner O'Connell-Diaz has served as Chair of the Chicago Bar Association's Public Utility Law Division.

In April 2003, Governor Rod Blagojevich appointed her to the Illinois Commerce Commission. She was reappointed in 2008 to a second term. She currently serves as the chairman of the ICC’s Water and Transportation Committees. In addition Commissioner O’Connell-Diaz serves on the National Association of Regulatory Commissioners’ Committees on Electricity, International Relations, and Water.

Commissioner O'Connell-Diaz is married and resides in Bull Valley, Illinois. She is also active in her community serving on the board of the Bull Valley Association, Board of Trustees for Regina Dominican High School and a lifetime member of the Naval Reserve Association.