Armitt has a track record in international construction, civil engineering and transport, both in the public and private sectors, most recently as chairman of the Olympic Delivery Authority.
He is currently deputy chairman of Berkeley Group and is a member of the Transport for London Board. He was chairman of the Engineering and Physical Science Research Council until 31 March 2012.
From 2001 to 2007 Armitt was chief executive of Network Rail and its predecessor, Railtrack. In 1997, he was appointed as chief executive of Costain, a position he held until 2001.
Before this he was chief executive of Union Railways, the company responsible for development of the Channel Tunnel Rail Link, and previously spent 27 years at John Laing plc rising to become chairman of the Laing International and Civil Engineering Divisions.
Armitt will initially join the National Express board as a non-executive director on 1 January 2013 and then take over as chairman on 1 February.
John Devaney has been chairman of the coach operator since April 2009 and announced in July his intention to retire from the board to focus on his other business commitments.
"I am delighted to welcome Sir John as our new chairman. His exceptionally strong international track record and extensive knowledge of the UK transport sector meant he was an ideal candidate for the role," said senior independent director Tim Score, who led the recruitment process.
"I would like to thank John Devaney for his significant contribution as chairman, in particular his critical role in turning the company around and establishing a strong base from which to generate profitable growth. Sir John will now oversee this next phase as we continue to deliver operational excellence while developing a new pipeline of opportunities across our portfolio in the US, Europe and North Africa to drive medium-term growth."
Armitt added: "I am very pleased to join National Express Group as its chairman, a company that is respected internationally for the quality of its services. I would like to pay tribute to John Devaney for his chairmanship of the business and look forward to building on his successful tenure in the coming years."