The firm beat Wates to take the project last October after chopping the original cost estimate down from £26m to £18m.
Thetford Academy was one of the new wave of projects released by Partnerships for Schools after the Government spending review.
Mansell will build a mix of new three-storey and refurbished buildings for around 2,000 pupils gathering school buildings onto a single site for the start of the 2013 new term.
The new building will include a library, IT suite and specialist areas for design technology, science and music.
A new sports hall and a separate sixth form centre will also be constructed, along with refurbishment to the existing north campus.
A spokesman for Norfolk County Council added it would work with its property consultants to determine the future of the old school south site which it anticipated would no longer be needed by the council.
She added: “There has been local interest in retaining the swimming pool but the technical and financial implications of this are yet to be investigated.”