Though no formal pitch took place, Eurostar's other roster agencies, Fallon in the UK and TBWA\Brussels, were in contention for the brief, which encompasses the company's marketing activity before and during the Games.
Leg, which already works with Eurostar in France, was appointed following Eurostar's unveiling as the official London 2012 international rail provider last May.
The deal involves Eurostar as a tier-three sponsor of the Games and allows it to use London 2012 branding.
Fallon's most-recent UK ad campaign for Eurostar was called "exploring is beautiful" and followed a young girl as she travelled around a city attempting to speak to animals.
Eurostar called a media planning and buying review last October, which is ongoing. Vizeum is the incumbent on the media account.