Ben Nunn will provide strategic advice to Lexington's clients on politics, the media and campaigns.
He will work with the 80-strong agency’s leadership team, which includes former Conservative No. 10 press secretary, Paul Harrison.
The appointment will mark a return to agency life for Nunn, who has spent the past two years at the top of UK political comms.
Nunn helped Sir Keir Starmer win the Labour Party’s leadership contest as his personal comms chief last year. He had previously been part of Starmer’s comms team when he was shadow Brexit secretary under Jeremy Corbyn’s leadership.
But he resigned in June, following Labour’s poor performance in the Chesham and Amersham by-election – in which the party came fourth.
In his resignation letter to colleagues, he told them it was a difficult decision but a chance to do something different.
Nunn started out as a policy and campaigns intern at children’s charity Barnado’s before working in the House of Commons as a parliamentary researcher for two years.
A three-year stint at MHP Communications followed, before Nunn moved to Incisive Health as an account director. He left the agency to join Labour’s health team under then-shadow health secretary Heidi Alexander.
Lexington said Nunn’s appointment follows on the heels of a raft of other senior hires, including a head of responsible business, a public affairs and comms director, and its first head of people and culture.
Mike Craven, founder of Lexington, said: “Ben has been a key player at the highest levels of the Labour Party for the last five years but he is also a very experienced communications consultant and he is widely respected by business as well as by the journalists and politicians he has worked with.”
Nunn added: “I'm delighted to be joining Lexington, which has a proven track record of delivering positive change for its clients. This is an exciting time for the company and for its client, which – more than ever – depend on first-class strategic advice.”
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