Daubney Services will operate alongside Mowlem's M&E business Dudley Bower Services. Daubney's managing director, Martin Daubney, is staying with the company.
The firm, which has a turnover of £20m, specialises in defence contracts. The acquisition is part of chief executive John Gains' strategy to develop Mowlem's services business.
He said: "Daubney strengthens our M&E capability both geographically and in terms of competency. Strategic acquisition over the past two years has given Mowlem the capacity to offer integrated solutions."
Gains added that Mowlem was still looking for other acquisitions to strengthen the group's range of services.
In August Mowlem bought cleaning firm Pall Mall for £42.3m; it also acquired railway group Fasttrack in May.