In his new role, Gillespie will oversee agency relationships for the online search company in the UK market.
A Google spokesman said the hiring reflects the importance of agency relations to the search giant. He said: "We are investing in our teams, including how we help agencies maximise the value they give to their clients from the advertising platforms we offer."
Google refused to comment further on the hiring, but said it plans to make more senior appointments this year in order to build its agency team and strengthen its relationships with the agency community.
The move comes as Google looks to cement its market- leading position, following the launch of adCenter, Microsoft's search product, last year.
Gillespie was a founding director of Omnicom's pooled buying unit OPera. He left in July 2005 and worked in a number of consultancy roles, including as the project director of UK Music Week, the Radio Advertising Bureau's UK music promotion.