Headstream will hold the McLaren Automotive account on a retained basis following a competitive pitch against other digital PR specialists.
Williams declined to reveal the value of the retainer.
A key task for the agency will be this month's launch of the F1 giant's high-performance sports car, the MP4-12C.
McLaren Automotive sits in the McLaren Group as the road car division alongside its F1 counterpart.
Kate Williams, director at Headstream, said: 'This is not just any car manufacturer, it is an innovative pioneer of high-performance sports cars and, as part of the McLaren Group, it boasts incredible automotive heritage.'
The agency worked with McLaren last year on a sentiment monitoring brief surrounding the first announcement of the new MP4-12C.
Mark Harrison, head of PR at McLaren Automotive, said: 'The first full unveiling of the MP4-12C to world media and the launch of McLaren Automotive across the globe are hugely significant in the automotive industry. Headstream will be essential to our PR strategy in delivering these important messages to influential digital and social media.'
Headstream has won a series of digital PR and social media accounts, with the BBC, Cadbury and TomTom.