Cameron's former deputy chief of staff, Baroness Kate Fall, is to join Brunswick to serve as a senior partner.
Fall was part of Cameron's team until she departed office last summer, with the former Prime Minister also nominating her for a life peerage in 2015.
She becomes one of a growing number of former Cameron staffers to become lobbyists.
It emerged that Cameron's former communications director Sir Craig Oliver joined US consultancy Teneo Blue Rubicon in November last year.
Similarly, Cameron’s former director of strategy Ameet Gill teamed up with Vote Leave's Paul Stephenson to launch Hanbury Strategy in September, and Cameron's press chief Graeme Wilson joined financial services lobbyist Tulchan Communications in late November.