Simon then served as an assistant United States attorney of Northern District of Indiana from 1990 to 1997.
Simon served as an assistant United States attorney of the District of Arizona from 1997 to 1999.
He was an assistant United States attorney and chief of the criminal division of the Northern District of Indiana from 1999 to 2003.
He was confirmed by the United States Senate on March 27, 2003, and received his commission the same day.
[2] On September 26, 2008, President George W. Bush nominated Simon to a seat on the United States Court of Appeals for the Seventh Circuit vacated by Judge Kenneth F. Ripple, who took senior status on September 1, 2008.