McBrien was at New Look for more than two years. Before his tenure at the clothing store he was director of ecommerce at Arcadia and, before then, head of online marketing for Microsoft Xbox EMEA.
He will report into Laura Wade-Gery, executive director of multichannel ecommerce, , where she was chief executive.
McBrien will be tasked with helping M&S achieve its target of increasing online sales from £500m to £800m-£1bn by 2014.
As part of the sales drive, to oversee a complete revamp of its online presence.
Yesterday, M&S launched a competition to look for a presenter for its online TV channel M&S TV.
New Look is also pushing its multichannel activity and last month announced a "click-and-collect" service, available at 600 stores.
Today it launched an iPhone app, developed by Mobile Interactive Group. The app offers fashion and lifestyle content, as well as style advice and access to the mobile commerce site.
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