Horrocks, who has been editor for 12 years, has decided to set up his own media agency called Essential Communications.
He joined MEN Media from the Daily Mail in April 1975, working as a reporter, crime correspondent, news editor, assistant editor and deputy editor before being appointed editor in 1997.
He is one of the longest-serving regional editors in the UK.
MEN deputy editor Maria McGeoghan will be appointed acting editor during the process to appoint Horrocks' successor.
Horrocks said the agency would be targeting business media companies to offer comms, internal comms and crisis management support.