Collison has been promoted following the resignation of former MD David Hay.
A company spokesman said: “After five years as MD, David Hay felt the time was right to make a career move to focus on other opportunities being created by the ever-changing public sector landscape.”
Ryhurst specialises in land and asset solutions for the health and social care markets.
Collinson has been with the firm since 2008 as Business Development Director. He will also join the Rydon Group Board.
Bob Bond, Chief Executive of Rydon said: “Stephen was an obvious choice for the role of Managing Director, bringing an unrivalled breadth of experience and capability at what is an exciting time for Ryhurst.
“On behalf of the Rydon Group Board, I am delighted to welcome him to his new role and look forward to him continuing to deliver first-class estates and facilities services for our current and future NHS partners.”