The 52-year-old Simpson announced his candidacy Monday as a Republican.
Democrat Jim Hood, 48, is seeking a third term as attorney general. Hood was a district attorney in north Mississippi before winning statewide office.
Simpson worked six years as an assistant district attorney in Harrison County before Democratic Gov. Ronnie Musgrove appointed him to a nonpartisan job as circuit judge in December 2000.
Republican Gov. Haley Barbour appointed Simpson as public safety commissioner in April 2008.
Simpson says he'll resign as commissioner during the campaign, but he's not sure when.