Project manager Nigel Barnes has been promoted to group managing director and quantity surveyor Cathy Bousie will become group commercial director.
Macob chairman John Roberts said Barnes had extensive experience of the industry and was capable of tackling prestigious projects in the public eye.
He said: "Barnes has the drive and determination to win new business and deliver existing projects on time and within budget."
Barnes was previously project manager for the St David's Hospital PFI scheme in Cardiff.
Roberts said Bousie had a background in main contracting and would be responsible for new business.
The Macob group turns over £75m with a profit of £2m.