Engineers cut the masts free during a daring project, initially estimated to cost £9.5m. A crane supported the poles as they were sliced off and lifted to safety.
Mather said despite the “unique risk and a challenging site”, the project was completed ahead of schedule and below forecast costs.
The BBC has asked the Department for Transport for details of the final costs.
SS Montgomery was loaded with 7,000 tonnes of munitions in 1944 and directed to anchor off Sheerness, Kent, but grounded on a sandbank.
Much of the cargo was removed, but the wreck remains in the sea and is visible about five miles away (8km) in Southend-on-Sea on a clear day.
Chatham Historic Dockyard will conserve the masts, following a campaign for them to stay in Kent, as they technically remained property of the US government.
The wreck remains subject to continuous monitoring and control via the exclusion zone in place.
