Fareham, a charted structural engineer, has been with the top-five steelwork business for 46 years.
He became managing director of the Billington Structures in 1991 and has headed the main AIM-listed Billington holding company for the last six years.
Chairman Peter Hems said: “Stephen has been an integral part of this company for many years and has made a significant contribution to the success of the business and is well respected in the wider business community.
“We thank him for that contribution and wish him well in his retirement. A further announcement will be made in due course as regards his successor.”
At the end of May, Billington appointed Mark Smith, former London operations director of William Hare and past director of D. A. Green Steel and Cladding, as group chief operating officer.