The agency won the account following a final shoot-out against Tesch, which launched last year in London.
Dare, Poke and Dentsu London had also been invited to pitch for the business, but were eliminated at an earlier stage of the review process, which kicked off in February.
LBi will handle the digital duties for the Xperia range, including the new Play handset, as part of a wider brand awareness campaign.
The agency is expected to work with Sony Ericsson on a move away from promoting the functionality of the handset to focus on building a brand for Xperia.
Sony Ericsson unveiled its Xperia Play in March, a smartphone designed specifically for gamers, which which draws on features from Sony's PlayStation console. It debuted at the Mobile World Congress in Barcelona, and will be available from next month.