The account was previously handled by MindShare's WPP stablemate Mediaedge:cia. MindShare pitched as a Group M agency against Carat, Napster's media agency of record in the US.
Napster, which is planning to increase the share of its spend allocated to online, briefed agencies to devise a strategy to drive new users to its website. The pitch was led by MindShare Interaction, MindShare's digital media specialist.
MindShare will plan and buy across all media channels and develop a communication strategy, centred on online but also using display, search and affiliated technology.
Jo Lyall, the managing partner at MindShare Interaction, said: "Napster is a fantastic addition to our portfolio, a truly online business which is open to new ideas and ways of thinking about media.
"We are really looking forward to working with it at a time when the music download business is exploding."
Napster originally launched in May 1999. It achieved notoriety as the first free music download site and became embroiled in constant legal battles with the record labels over copyright infringement.
It closed in July 2001, but relaunched in 2003 as a legal download site, having been bought by the digital media company Roxio.
Napster is one of the most popular music download sites, second only to Apple's iTunes. It allows members who sign up to download more than two million songs to their PCs.