Sir Alan Thomas, chairman of Firth, and Tim Wade, chief executive of Hyder, will head the board. Other board members are financial and commercial director Simon Hamilton-Eddy and non-executive directors Peter Morgan and Peter Higgins.
Wade said the deal was the next step in the development of Hyder and would give the group access to investor funds for acquisitions and organic growth.
Firth was the holding company of a steel manufacturing and property development group until its holdings were sold in 1999. Thomas said Firth had been looking for a suitable company to invest in and Hyder had met the criteria.
Hyder was the subject of a £20m management buyout in January last year from American utility group Western Power.