The firm beat Wates to take the project with a bid price of around £18m compared to the original £26m cost estimate.
The Thetford Academy is one of the new wave of projects released by Partnerships for Schools after the Government spending review.
An application for detailed planning will be submitted shortly. The plans will consist of a mix of new and refurbished buildings for around 2,000 pupils and must be finished in time for the start of the 2013 new term.
Mansell will also build outdoor sports facilities.