He will become a director of its London corporate reputation practice. Kemp is a former journalist and was head of media for the Lib Dems during the 2010 election.
He spent four years in Number 10 as deputy head of press between 2010 and 2014 before exiting to the private sector, where he worked at Blue Rubicon, Open Road and more recently the National Grid.
Kemp continues to provide political commentary on the likes of Newsnight, The Today Programme and Good Morning Britain.
Kemp said: "MSL’s ambitious plans and integrated approach were hugely appealing to me as I looked to return to agency life. The nature of the company and diverse network of clients mean this will be a very exciting time to join the team."
He joins senior directors Tom Bell and Simon Harrison in spearheading the growth of MSL’s corporate reputation offer, expanding its public affairs and media capabilities and further reinforcing its crisis services.
James Warren, MSL’s general manager, said the agency was excited to see Kemp join. "His addition augments a strong set of senior consultants and will help drive continued growth across our portfolio."