All three incumbents were invited to re-pitch for the business, but today's move ends Mars' media relationship with the latter two holding groups.
According to Andrew Clarke, chief marketing and customer officer at Mars, GroupM clinched the pitch because it created "a custom operating model" for the Malteasers owner.
Clarke said the new approach will let Mars put data "at the heart of its decision making" as well as drive speed at a global, local and campaign level.
The FMCG giant kicked off the pitch process at the start of the year and the business said that GroupM will take over all proceedings in January 2019.
Clarke added: “This partnership will be a crucial accelerator in our ambition to be quicker, bolder and even more innovative when it comes to meeting our consumer needs. It brings thought leadership and actionable use of data and insights to meet our media needs."
The move marks another big win WPP's GroupM arm, with its agency MediaCom having recently retained Shell's multi-million dollar account. Earlier this year, the network was left reeling after Mindshare lost HSBC's $400m account to Omnicom's PHD.
GroupM is poised to face some challenges in handling Mars' media account. Earlier this week, the confectionary firm paused all of its global ad spend with YouTube over brand safety concerns.
The decision was taken after a pre-roll ad for Starburst was shown before a video said to be fuelling London gang violence.
The spot had been loaded into an upload of a film from Moscow 17, a drill music group. Drill is characterised by gritty, violent lyrics and has been cited as the soundtrack to ongoing gang warfare in London, as rival groups often use music videos and tracks to provoke one another.
Mars, which has put its money where its mouth is previously to prove its intentions to tackle ad misplacement, said: “This clearly breaches our brand safety guidelines and Mars adverts should never run alongside such content.
"We have taken the action to remove all our online advertising on YouTube and can confirm we are working with Google and our media buying agencies to understand what went wrong."