Lee replaces Paul Constantine, who had been Newcast's managing director since October 2010. He took over from Mark Waugh, who is now the global managing director of Newcast. Waugh had led the UK division from its inception in 2007.
Constantine will continue in his role as managing partner of ZenithOptimedia, focusing on business growth across clients including Comparethemarket.com.
Lee, who has worked for the agency for 11 years in roles including client services director, new-business and marketing director, will take up the position immediately and continue to report to Gerry Boyle, the UK chief executive of ZenithOptimedia.
In his new role, Lee will be responsible for continuing growth and development at Newcast, which has grown to billings of more than £45 million and a team of 22 since its launch.
Content that Newcast has produced on behalf of clients includes B&Q's long-running sponsorship and content partnership with 4Homes and O2's live streaming of a Kasabian gig on YouTube.
Boyle said: "I am delighted that Christian has accepted the challenge of continuing the momentum we have in the content space and leading Newcast into the future."