As well as bolstering the input from the Australian strategist, the contract secures polling services from his business partner Mark Textor and support from a team of staff for the 2015 poll.
The Prime Minister recruited the man credited with Mayor of London Boris Johnson's ballot box successes last year to work part-time on boosting Tory re-election prospects and the refreshed deal signals that the party is shifting its 2015 preparations up a gear.
A senior Tory said: "We are thrilled to have Lynton. Labour hate him because they know he gets results."
Mr Crosby was closely associated with the Tories' failed crime and immigration-centred 2005 campaign under Michael Howard and former deputy party chairman Lord Ashcroft warned he was a "recipe for conflict and confusion".
But Mr Johnson urged Mr Cameron to "break the piggy bank" to get him on board.
Mr Crosby has since been the focus of media scrutiny amid claims his lobbying firm posed a conflict of interest with his role as a key aide to Mr Cameron.