The firm won the contract through a competitive tender process, having previously worked with Hammersmith and Fulham.
Glendale will now employ extra staff to carry out the contract, according to a representative.
The contract will involve carrying out the cyclical maintenance programme of the local authority's street tree stock. The council has a number of street trees that are more than 150 years old and require regular pruning and associated work. The operation will be managed from Glendale's Watford depot, which is also home to the firm's supply arm, Civic Trees.
Hammersmith and Fulham is expected to benefit from funding from London mayor Boris Johnson's city-wide tree planting scheme in the coming months.
"Glendale Countryside has been undertaking tree maintenance for Hammersmith and Fulham on an ad hoc basis for a number of years," explained regional director Jessica Ward.
"We are now looking forward to developing our partnership through this fixed-term contract." The contract began on 1 December and will run to 2011.