Gavin Jones, which does landscape maintenance at Kensington Palace, Marlborough House and Buckingham Palace, scooped grounds upkeep work for Aldershot and Salisbury Plain.
The five-year contract is worth nearly £10m and Gavin Jones won a highly competitive tender with several rival companies involved, said business development manager Yvette Etcell.
Sites are grouped into four garrisons including the 500ha Aldershot site, described as the home of the British army. Total area of greenery across all sites is around 1,200ha.
Etcell said: "Mobilising the contract involved the transfer of almost 50 staff from two previous contractors and the procurement of new plant and kit to the value of almost £700,000."
Aldershot has first-rate sports fields and the British Olympic team would use it as a base for the 2012 Games, said Etcell. Aspire Defence Services awarded the contract on behalf of the Ministry of Defence.
Gavin Jones also scooped a grounds-care contract for the 1930s Grade II-registered Stoke Poges Memorial Garden, in Buckinghamshire, worth around £1.2m.
Etcell said the two contracts together would increase the firm's grounds upkeep turnover by around a quarter from £6m to £8m.
and Gavin Jones won a highly competitive tender with several rival firms, said business development manager Yvette Etcell.