Lansons stole the business from incumbent Hill & Knowlton following a four-way pitch last week. The brief includes corporate, trade, consumer, digital and public affairs.
H&K had worked for LinkedIn for a year and a half and fought to keep the business last week. The Red Consultancy and CMP Communications also took part in the pitch.
With 40 million users, LinkedIn is significantly smaller than Facebook, which has 200 million users. But it remains well ahead of challenger brand Twitter, which has 14 million users.
Before H&K, LinkedIn hired Chameleon in November 2007.