The agency won the business following a pitch against a selection of existing Unilever roster agencies, including AKQA, the incumbent on the account, and Cake.
Lean Mean Fighting Machine will now work on a number of digital projects for the brand to build on the advertising that has recently supported Sunsilk.
In September last year, the brand appointed the WPP-backed Argentinian agency Santo as the lead creative agency on the global account. Recent campaigns for Sunsilk have focused on the brand's music ties.
Last year, Sunsilk secured a deal with Madonna to feature in its "life can't wait" spot.
The ad, which also featured Girls Aloud, showed some of the most iconic moments in pop music, and highlighted how each singer's hairstyle played an important role in their rise to fame.
The campaign was supported by digital work that encouraged consumers to visit a website and share their own iconic moments.