The agency will work on a number of briefs for Orange across its business services brand including web build and web design work as well as creating a series of digital ad campaigns.
Proximity London joins AKQA, Chemistry, BD Network and Poke on the mobile phone operator's digital roster.
Fallon, the company's creative agency, has also created some digital campaigns for Orange in recent years.
Earlier this month Proximity London made three senior digital creative hires to bolster its online credentials.
It appointed a digital creative team from ad agency Rainey Kelly Campbell Roalfe/Y&R, award-winning writers and art directors Richard Glendenning and Simon Parkin.
In January Orange revamped its Orange Wednesdays film site making it part of a film portal with a film finder and video content.
The site offers news, features, celebrity interviews and picture galleries, and highlights the two-for-one Orange Wednesdays cinema offer.