He will join the McGuire Woods law firm handling government investigations/white collar criminal work dealing with regulatory agencies.
Willoughby told ABC11 he delivered a letter to Governor Pat McCrory Thursday morning. It will be up to the governor to decide to appoint someone to manage the prosecutor's office during the election process.
Willoughby first said in January he would not run for reelection after 28 years in office.
He was appointed acting district attorney by then-Gov. Jim Hunt in 1983. He was first elected in 1986 and was sworn-in in 1987.