Armitt spent 27 years with John Laing, on various projects including the Sizewell B nuclear power station rising to become the chairman of Laing's International and Civil Engineering Divisions. In 1993 he was appointed chief executive of Union Railways, the company responsible for implementing the Channel Tunnel rail link. From 1997 to 2001 he was chief executive of Costain Group, which he converted from an annual loss of £62 million to a profit of £6.5 million. In 2001 he became chief executive of Railtrack, and from 2002 to 2007 its successor, Network Rail,