The five-year contract with Central Networks — part of E.ON — is a renewal of an existing five-year relationship with the firm.
The deal encompasses the management of trees and other vegetation along Central Network's overhead electricity transmission and distribution network throughout Worcestershire, Gloucestershire and Herefordshire.
The contract has the potential to rise to £30m over the five-year term, which begins in January.
Connaught Environmental managing director of utilities Richard Jowett said: "Millions of homes and businesses rely on the electricity that Central Networks distributes and we take the responsibility for helping secure this supply extremely seriously.
"The fact that we have won this extension following an open tender process is a strong vote of confidence in the professionalism, safety quality and cost effectiveness of our service delivery.
"We are delighted to have retained this contract for another five years and we are looking forward to continuing our relationship with Central Networks," he added.
Connaught bought grounds maintenance, arboriculture and utility contracts firm Fountains in July this year. Fountains had been working with Central Networks on its tree maintenance contracts. Central Networks has 133,000km of underground and overhead cables nationwide.