The process started last September, and included the incumbent, The Piper Agency. The final shoot-out took place between MPG, MediaCom and Initiative.
The review, which was managed by the AAR, came two months after National Express switched its creative account from the incumbent, Inferno, to Hooper Galton.
The remit includes planning and buying for National Express coaches and the company's train and bus divisions.
Media Contacts, MPG's digital planning and buying agency, has worked with National Express since 2005. John McLoughlin, its managing director, will take overall responsibility for the account.
McLoughlin said: "Everything we've done for National Express has been rooted in accountability, understanding which channels are the most valuable, and then ensuring we get the most out of them.
"We're really proud to get this opportunity to extend our remit across all media channels and drive the respective businesses forward."
The National Express coach network spans the UK and mainland Europe. It also runs intercity, commuter and rural passenger train services and urban bus services in Birmingham and the West Midlands.