Mitchell-Rose was one of Osborne’s key advisers from 2006 until last November when she moved to Washington.
She joins Freud’s corporate reputation practice this week as an associate board director, working on comms strategy and issues-based campaigns.
It is her first agency role. She started her career in journalism for the BBC and Reuters before becoming media adviser to then Conservative shadow health secretary Liam Fox.
The Spectator called her Osborne’s ‘fixer and gatekeeper’ when she stepped down.