McTernan will work with Sean Worth and Nick Faith, who founded the agency in 2014, offering senior strategic comms advice to clients.
Following his stint as one of Blair’s senior team, McTernan was a special adviser to Des Browne, the then Secretary of State for Scotland and Secretary of State for Defence, and then to Jim Murphy while he was Secretary of State for Scotland, between 2008 and 2010.
He has also worked as director of comms for the Australian Prime Minister, Julia Gillard, before returning to the UK to become the chief of staff to Murphy during his time as leader of the Scottish Labour Party.
Sean Worth, director of WPI, said: "John has one of the most strategic minds in political communications. He has a wealth of experience advising some of the most senior decision makers both in the UK and across the world. We are thrilled that he is joining our growing team."