Kelly spearheaded the redesign of the Mirror website, Mirror.co.uk, and last year oversaw the launch of 3am.co.uk and MirrorFootball.co.uk.
Perform creates and manages digital media channels for a number of sports. It runs video-on-demand for 79 football league clubs, as well as selling ads for PremierLeague.com.
Kelly will be managing director of a new publishing division at Perform, responsible for building new audiences and revenue streams. He will report to Simon Denyer, joint chief executive.
Kelly has held a number of roles at MGN, including associate editor of the Daily Mirror, head of features, and technology columnist. He believes newspapers online offerings should rely less on search engine optimisation (SEO) and more on journalism.
His boss, Sly Bailey, chief executive of Trinity Mirror, also believes that building loyal audiences is more important than SEO.
Mirror Group Digital's portfolio of sites – which includes Mirror.co.uk, 3am.co.uk, Mirrorfootball.co.uk, Dailyrecord.co.uk, Sundaymail.co.uk and People.co.uk – is a relative minnow compared to rival national newspaper sites.
According to ABCe results for August, Mirror Group Digital registered 499,645 average daily browsers.
Mail Online, meanwhile, registered 2,552,511 while guardian.co.uk registered 1,941,540.