McNeel, 47, will be based in TBWA's worldwide headquarters in New York, overseeing Masterfoods brands including Mars, Whiskas and Uncle Ben's.
TBWA took on the business in May, replacing D'Arcy as Masterfoods' third roster network.
Before taking up his position at Saatchis, McNeel worked in the Middle East as the chairman and chief executive of the network's eight offices in the region. He was also an account director for clients such as Johnson & Johnson, Visa and British American Tobacco at local, regional and global level.
TBWA's president of the Americas, Tom Carroll, said: "John is a motivated and experienced leader with some of the best brands in the world. He is a tremendous addition to our network and his understanding and ability to build global brands will make him an indispensable part of the Masterfoods team."
McNeel added: "I've worked a long time overseas and I've got a good international grounding. The opportunity to work on Mars was too good to miss, and TBWA is a great network."
TBWA responded to being appointed to the Masterfoods roster by resigning its Cadbury business across Europe to avoid conflict with Mars. The network handles the brands across North America, Europe, Asia and Australia.
McNeel began his career as a copywriter, working in Paris and New York.