Zenith retained the estimated £90m UK account. Starcom won Germany and India from MediaCom and retained China and South East Asia.
The German account, in particular, is thought to be a significant win and roughly on a par in size with the UK.
MediaCom got some consolation as it won Australia and New Zealand from Starcom.
Zenith, which beat MediaCom and Dentsu Aegis Network’s Amplifi in the UK pitch, will be relieved to retain Mars after losing another longstanding client, Toyota, only last month.
WPP’s MediaCom has had a tough year. It lost Volkswagen Group’s estimated £2bn account to Omnicom’s PHD.
Mars continues to use MediaCom for its media planning on a global basis.
The US-based chocolate and pet food manufacturer is one of the world’s biggest advertisers.
Its media arrangements are relatively unusual as it handles media-buying through agencies on a local basis but manages its media planning account separately on a global basis.
A Mars spokesman said it carried out the media review, "to ensure that we have the strongest possible media-buying agencies supporting our business objectives around the world" and it evaluated agencies "on both qualitative and quantitative criteria".
The spokesman said: "We’re delighted to continue our partnerships with both Mediacom and Starcom as we continue to drive performance and innovation through our media agenda."
The review covered seven markets: Australia; New Zealand; China; Germany; India; South East Asia and the UK.
There was also a review in Japan that finished earlier and involved other agencies, according to Mars.
Publicis Media and MediaCom declined to comment.