The agency will provide political and stakeholder communications after winning a keenly fought four-way pitch. It is believed that Bell Pottinger, Weber Shandwick and Grayling were the other contenders.
The airport is launching a planning application imminently and will use the agency to make the case to a wide range of stakeholders including local community, NGOs and politicians.
Hill & Knowlton's account is said to be well over six figures and the agency has been retained until the end of the year.
The account will be led by head of corporate and PA, Tim Fallon. It is the first time the airport has used an external agency for public affairs work.
The hire of Hill & Knowlton comes in the week that BAA - which does not own City, but seven other UK airports - was criticised by the Competition Commission.
BAA is now planning a proactive media onslaught after its airport monopoly was called into question by the commission.
The airport group's deputy director of corporate and public affairs Malcolm Robertson said BAA planned to 'make itself more available and be as open as possible', fielding new CEO Colin Matthews for TV interviews and newspaper profiles.
But Robertson insisted that BAA's recent T5 woes, coupled with the commission report, did not constitute a reputational crisis.