Mitchell-Rose was one of Osborne's key advisers from 2006 until last November when she moved to Washington.
She joined Freud's corporate reputation practice this week as an associate board director, working on communications strategy and issues-based campaigns.
It is Mitchell-Rose's first agency role. She started her career in journalism at 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.
This story originally appeared on the website of PRWeek UK, the sister publication of PRWeek at Haymarket Media.