The agency was appointed after a seven-way pitch involving Soul, Vallance Carruthers Coleman Priest, Proximity London, KLP Euro RSCG, Joshua and Wunderman.
Maher Bird's brief is to help Yahoo! develop online advertising as a medium. It will operate alongside Yahoo!'s marketing and sales team by targeting key disciplines within advertising and media agencies as well as client organisations.
Trade marketing is a new area for the portal. Having a dedicated agency focusing on this sector is part of the brand's move to establish the internet as the sixth medium.
The Yahoo! UK & Ireland marketing director, Yonca Dervisoglu, said: "We chose Maher Bird because of the trade positioning it developed and the creative execution, as well as for the team it put behind this."
Yahoo!'s £2 million media planning and buying will continue to be handled by Manning Gottlieb OMD.
Earlier this month, the online brand parted company with Black Rocket Euro RSCG, its US advertising agency of nine years.
A new WPP Group division, Soho Square, was subsequently appointed to handle brand advertising. The bulk of Yahoo!'s £70 million global advertising spend is now expected to be created by Soho Square.
Yahoo!'s main rival, AOL, is also currently reviewing its £20 million creative account. The portal appointed cdp-travissully to its business-to-business account earlier this year.