The work is part of a package of improvements being carried out by Cornwall County Council to improve links between the islands and the mainland.
Kier’s work includes demolition of an existing sea wall and construction of a 23-metre quay extension.
Cornwall County Council published a contract notice for the jobs last December but said it received no appropriate tenders in response to the original procedure. The contract with Kier was therefore negotiated without a call for competition. However, it still came in marginally below the original £9m to £12m estimate, at £8,971,547.
The St Mary’s Quay extension is expected to be complete by March 2015, ahead of next year’s summer season.