Ian Kirby was assistant editor (politics) from 2006 until 2011, when the paper was closed in the aftermath of the phone-hacking scandal.
He will become a director with a broad brief to 'advise at a senior level on a range of media relations and campaigning work'.
During his tenure at NotW, Kirby interviewed four Prime Ministers and helped establish the paper's digital strategy as a member of the paper's senior management.
Kirby said that he is 'looking forward to developing new campaigns and media strategies' for clients.
The news follows a pledge made by MHP chief executive Gavin Devine to invest significantly in the agency's team in order to challenge the top five agencies.
Kirby's PR move follows that of his deputy, Jamie Lyons, who joined Tetra Strategy in October 2011.
NotW closed last July, after which agencies reported a surge in enquiries from its journalists seeking jobs.