The privately owned operator of the high-speed rail link between London's St Pancras station and the Channel Tunnel had used M&N Place for both briefs since 2011.
Pagefield will look to position HS1 as a key voice within ongoing debates around rail infrastructure in the UK, as well as running the press office and handling crisis comms.
HS1's owners support the creation of HS2, the planned high-speed line connecting London to Birmingham and then to Liverpool and Manchester, and are hoping for a link between the two lines.
The High Court recently ruled the Government's consultation on compensation plans for those affected by HS2 was 'so unfair as to be unlawful', but transport minister Simon Burns said the case had, in effect, given the 'green light' to the project as the Government won nine of the ten points of contention.
Pagefield founder Mark Gallagher said: 'HS1 is a vital part of the success story of British infrastructure, and a key driver for growth of the UK.
'Our role will be to make sure that it obtains the recognition in the media, Westminster and more broadly that it deserves.'
Consumer comms efforts will involve promoting St Pancras International, which is owned by HS1. This includes the public art installation Terrace Wires, which has replaced the Olympic Rings.
Blue Rubicon partner Chris Jones said work would focus on making the station a 'London landmark and a destination in its own right.'
International work will involve raising awareness among tourists of the benefits of using HS1 to reach London.
HS1 CEO Nicola Shaw said: 'The new comms teams led by Pagefield and Blue Rubicon will help us as we embark on the next few years of the St Pancras story. We look forward to the benefits of their wide experience, fresh approach and new ideas.'