Napoleon was appointed Wayne County Sheriff after the position became vacant in 2009. He was elected to remain in the position in 2012 and was re-elected every four years since, most recently in 2020. He spent most of his career at the Detroit Police Department, starting in 1975 and worked his way up to police chief, assuming the position in 1998. He retired from DPD in 2001 and became Wayne County assistant executive in 2004. Napoleon ran for mayor in 2013, losing to the current mayor, Mike Duggan, by about 10 percentage points. Napoleon, the son of a minister, was a Detroit Public Schools student, graduating from Cass Technical High School, later earning his bachelor's degree from Mercy College of Detroit and his doctorate from Michigan State University.