As digital content director Kelly and his team will be responsible for the creation and development of the division's growing portfolio of digital properties, which includes Mirrorfootball.com.
Kelly led the development of that site and gossip website 3am.co.uk, which were both launched this year.
He will also work closely with the advertising and marketing teams to ensure that the division maximises revenues from the nationals digital business.
Mark Hollinshead, managing director of Trinity Mirror's nationals division, said: "Matt has proved he has the perfect blend of skills for this position as he manages to balance creativity with strategic insight and commercial acumen. This is a key appointment as we look to further enhance and develop our digital ambitions."
Kelly will continue to report directly to Daily Mirror editor Richard Wallace.