GA takes the brief in place of six-year incumbent Citigate Dewe Rogerson, which is believed to have made it to the final shortlist of three, which also included Brunswick.
AMEC investor relations director Neil Jamieson said the repitch completes an 18-month review of company advisers, which has also seen Deutsche Bank replaced by UBS Warburg as the engineering and construction giant's corporate broker.
GA will work on communicating AMEC's strategies, particularly its planned acquisition of French contracting and constructing firm Spie, in which it owns a minority stake.
'AMEC is approaching the end of a transformation and moving forward.
We will be working with our corporate advisers in ensuring that the company is better understood in its new guise,' said Jamieson.
GA London chief executive Neil Bennett leads the account team, reporting to AMEC director of corporate comms David Paterson.