Darren Ennis will join the agency as a director in its fledgling Brussels office.
A former Reuters journalist, Ennis has been Ashton's top media and strategic communications adviser since her appointment in December 2009. He is responsible for developing Ashton's EU foreign policy narrative.
MHP opened its Brussels office, headed by Alisdair Gray, in April 2011.
Ennis said: 'I look forward to helping clients understand the complexities of EU politics and identify where the opportunities lie.'