It is believed to be the biggest international media review that the family-owned confectioner has conducted, although it does not include its largest market, the US.
Mars spends about $1.7bn a year on advertising and is the seventh-biggest advertiser in the UK, where it has used Zenith for more than 20 years and spends approximately £90m a year.
The company is unusual because it uses separate agencies to handle planning and buying.
MediaCom won the planning business globally in December 2014 as part of a consolidation, while a number of agencies manage buying in local markets. Mars is not looking to consolidate its buying with a single agency internationally.
The review, which includes Australia and New Zealand, India, Japan and Southeast Asia, follows rising concern across the media industry about agency transparency, pricing and technological change.
Mars said it began the pitch process this month "to ensure that we have the strongest possible media buying agencies supporting our business objectives around the world".
It added: "Due to the significance of the markets involved, and in order to maximise project scale and efficiency, we decided to co-ordinate the pitch timings."
Mars expects to name "the winning agencies" later this autumn.