Norton will work across all Lego brands including pre-school products Lego Duplo and Lego City. H&K held the account for three years.
Lego PR and promotions manager Melissa Wallace said Norton would take over from H&K in January. It will run PR around Lego's return after a six-year absence to the 2006 UK Toy Fair, the four-day industry showcase taking place at London's ExCeL from 25 January.
Lego's profile at the fair will be crucial to demonstrate the relevance of its brightly coloured building block products in an age of electronic video games.
The company is to unveil its Exo-Force range, based on Japan's Manga comic books, and is relaunching its Bionicle range.
Lego also wants to reassert its brand following the sale of its four Legoland theme parks to US private equity firm Blackstone Capital Partners in July.