Parker has spent the last 12 months in the Foreign and Commonwealth Office (FCO), where he is also Johnson's director of political communications. While it is known for being well connected in the world of politics, Parker will become the only staffer at the agency to have previously been a spad.
In his new role, which begins in October, Parker will report to agency chief executive Oliver Foster.
Foster said: "We are delighted to welcome Liam to Pagefield. His experience working for two of the most recognisable and influential figures in the world of politics and finance means he is a strong addition to our senior team."
Prior to joining the FCO last year, Parker was head of press to the governor of the Bank of England, Mark Carney. Earlier in his career, Parker was official spokesman for the chief secretary to the treasury between 2007 and 2011, following a role as account executive at consumer PR agency Shine in 2007.
Pagefield, which specialises corporate PR, public affairs and campaigning, has clients including The Museum of London, AB InBev, and Airbnb.
The agency was founded by former ITV comms chief Mark Gallagher in 2010.