The brief - which is for all COI advertising across newspapers and magazines for Government campaigns - covers all buying, while planning will be done in partnership with the roster agencies for the separate communications planning brief.
Peter Buchanan, COI deputy chief executive, who oversaw the pitch, said it was a difficult decision.
"The proposals from the final shortlisted agencies were of an extremely high standard and we have seen enhanced value through different approaches to planning and buying," Buchanan said.
"All the agencies showed a high level of commitment and it was clear they understood COI's fit within the media landscape.
"We will now build on our relationship with MediaCom to ensure continued success at producing campaigns that are cost efficient and effective."
The original shortlist included Starcom, Walker Media and Initiative, as well as MediaCom, which has held the account since 2000.
The COI cinema buying contract, currently held by Universal McCann, is also under review with a result expected later this year. The Interpublic-owned agency is up against Carat, MindShare, ZenithOptimedia, Manning Gottlieb OMD and sister agency Initiative.
Meanwhile, an announcement on the result of COI's communications planning agency roster is expected in January. The incumbents include Naked, Starcom, PHD and The Ingram Partnership.
Earlier this year, Starcom lost the brief for the £70m COI television account to Carat, but was then awarded the £20m COI radio buying account taking it from incumbent OMD. The shift in the television business led to Carat poaching Starcom's television buying director Kevin West to head the account.
For all briefs currently under review, the government body has said it will be assessing shortlisted agencies on negotiation strategy, experience and quality of service.
Earlier this year, the COI renewed its creative roster. Agencies including Wieden & Kennedy and Clemmow Hornby Inge were appointed to the £156m account.
The COI also recently appointed OMD data to provide a new tool and service to improve campaign planning and response analysis. The system called COI Atremis will launch next year and will allow the COI and its clients to collaborate with OMD data to collaborate via a secure web portal.