He brings a wealth of infrastructure and transport industry experience from the UK and around the world, having led Amey for 15 years.
His appointment comes as HS2 chairman David Higgins starts the hunt to replace HS2 chief executive Simon Kirby, following his sudden resignation last week and a chief operating officer to strengthen the senior management team.
HS2 is gearing up to start building Britain’s new high speed rail network. Royal Assent for the London to Birmingham leg of the project is expected by the end of 2016, with construction set to begin in 2017.
Higgins said: “We are looking for leaders with experience of large and complex construction challenges to advise our work, and Mel certainly fits that description.
“He transformed Amey into one of the UK’s most successful engineering and infrastructure companies, and HS2 will certainly benefit from his considerable industry knowledge.
Elwell said: “HS2 is a truly exciting endeavour that has the potential transform the UK’s infrastructure industry.
“The opportunity to play a role in creating this legacy is too good to miss.”