The Oxfordshire-based firm will join Frosts Landscape Construction in the northern section of the Olympic Park in the coming weeks.
White Horse Contractors was appointed by managing contractor Bam Nuttall to undertake the east soft-landscape package; Frosts is to carry out work on the west package (HW, 25 September).
White Horse managing director Robert Donald told HW he expected at least 30 staff to work on site during times of "intense activity".
"The bid team worked tirelessly preparing the bid and deserves all the credit," he said.
"It was an awful lot of work. Part of our technical submission was based on the quality of our in-house staff, so we hope to resource as comprehensively as we are able to from our own team."
One of White Horse Contractors' main suppliers is Gavin Jones, and Donald said he expected to sub-contract to the Surrey-based firm. "There is quite a lot of organisation before we go live on site, but we expect that to be as soon as possible," Donald added.
Maintenance obligations mean that White Horse Contractors will be on the east London site until the start of the Games in 2012.
The contract includes installation of drainage, irrigation, completion of the earthworks, planting of trees and shrubs, installation of footpaths and terraced seating areas for a space that forms a major part of the new habitats being created across London's Olympic Park.
The east of the quieter northern section of the park will boast wetlands, ponds and swales, sloping planted riverbanks, meadows and wet woodlands.
Olympic Delivery Authority chairman John Armitt last week visited Salix Nursery in Norfolk to view wetland plants being grown for the park.
More than 30 species of native reeds, rushes, grasses, sedges, wet wildflowers and irises have been grown initially by Salix in its nursery on the Gower peninsular in Wales.
Around one-third were grown from cuttings and seeds collected in and around the Olympic Park before construction started in 2008. The plants are now being grown on in waterbeds and pallets in the nursery's Thetford site before being planted in the Olympic Park from next spring.