Mansell is building five training pools at Eton Manor alongside training pitches for a host of sports.
The site will also be a key Paralympic venue and provide new sports facilities for local people after the Games.
Initial ground work started in the spring and work is now underway on building the permanent sporting facilities, with the foundations due to be completed by the end of the summer.
John Armitt, Chairman of the ODA, said: “The start of work on Eton Manor is a significant milestone, the start of construction on the last permanent venue on the Olympic Park.
“Eton Manor will play a key role throughout the Games and leave behind a permanent facility which will benefit the local community for years to come.”
During the Olympic Games, Eton Manor will be an aquatics training venue, housing three Olympic-size temporary swimming pools, a temporary synchronised swimming pool and a temporary water polo pool.
During the Paralympic Games, it will host the Wheelchair Tennis competitions with temporary seating for 10,500 spectators.
After the Games, the permanent facilities will be transformed into a hockey centre with two competition pitches, a tennis centre with four indoor and six outdoor courts, and provision for five-a-side football pitches.