Johnson entered politics in Columbus, becoming Greg Lashutka's chief of staff after he managed Lashutka's successful 1991 campaign for mayor of Columbus. Johnson left that post when he was appointed to the Ohio Senate in 1994. After being elected to two terms and rising to the second highest post in the Senate, in 2001, Governor Bob Taft asked Johnson to join his cabinet as Director of the Ohio Department of Development. In 2005, Taft appointed him lieutenant governor. In December 2006, Johnson resigned a month before his term ended.