The agency said McTernan will provide senior political and comms advice across BCW clients and will work closely with Steve Hawkes, who recently left his role as deputy political editor of The Sun to join BCW London, starting in October.
Both McTernan and Hawkes report to Nick Williams, MD and head of issues and public affairs.
McTernan joined from PSB, a research and analytics consultancy that is part of BCW.
He is a former director of political operations for Blair, and before that worked with Gillard as director of communications. His other roles have included special adviser in the Scotland Office and the Ministry of Defence.
Williams said: "John’s insights and experience are widely recognised, not only in the UK but also internationally. By bringing in this talent to the BCW team, we are fast building the UK’s strongest and most experienced political, campaign and issues management teams."
McTernan said: "BCW’s political and campaigning strength in London is highly impressive and I am excited to join this team. It’s an extremely exciting, challenging and fast-moving environment where the political decisions of the day will impact every single business or investor with an interest in the UK."
Meanwhile, BCW's issues and public affairs team has published an analysis of what a Labour Government would mean for business, called A Risky Business? What Would a Labour Government Mean for You?