The move is being called a "mutual decision" between the two companies, but follows MediaVest winning the £9m media business for Global's rival Bauer Radio last year.
In October 2008, Nicola Thomson, former director of marketing for Global Radio's three brands, Heart, LBC and Galaxy, was promoted to group director of marketing across the whole company.
Thomson is a former marketing director of Emap's Magic, which is now part of Bauer Radio, and will be managing the review.
In March 2008, Bauer extended its relationship with Starcom MediaVest Group by handing it the media planning and buying business for its Big City network of stations.
The agency was appointed without a pitch and assumed control of the media account for the group's network of stations in England, Scotland and Northern Ireland in addition to the Magic, Kerrang! and Kiss FM accounts it has handled since 2001.
Media planning and buying for GCap Media used to be handled by Rocket, part of the PHD group.
MediaVest has worked across Global Radio's account including campaigns for Galaxy and LBC, and most recently the national rollout of the Heart brand.
A joint statement issued by Global Radio and Mediavest stated: "After a long and successful relationship between MediaVest and Global Radio (formally Chrysalis Radio), both parties have come to a mutual decision to part company.
"MediaVest is passionate about the success of radio and both parties wish each other success for the future."
Global Radio was formed from the acquisition of Chrysalis Radio and its brands Heart, Galaxy and LBC in July 2007, and GCap Media and its brands Classic FM, Capital Radio and Xfm, in June 2008.
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