Mouchel chairman Jim Harding said the company's certainty of approach and innovative route management were decisive factors in landing the deal.
The contract gives Mouchel responsibility for all 380 km of the M25, which for the first time will be managed in its entirety as one route. The firm will also manage roads that feed the M25, including the A1, M1 and M3.
Mouchel already manages the maintenance of emergency telephones and the cabling to electronic signboards, in partnership with the Highways Agency and the six police forces that administer the roads.
The only part of the M25 ring not controlled by the Highways Agency – the Dartford river crossing from Kent to Essex – is expected to revert to public ownership next year.
Mouchel has already called in consultants such as structural engineer Maunsell and street lighting expert Atkins Odlin to work on the project.
Mouchel also runs other stretches of road including the southern part of the M1 and the M6 in Cumbria.