Based in London and reporting to Craig Davis, JWT's worldwide chief creative officer, he will work across all of the agency's Unilever business including Knorr and Lux.
Over the past two years, Theophilakis has run S&J's creative department and oversaw the launch of its office in Amsterdam.
As well as handling the creative duties on the agency's flagship Mercedes-Benz account, he headed the pitch team that won the Olympus business across Europe.
Davis said: "Aris has a track record of creating big ideas for big brands across multiple markets and media. Those talents are precious."
Before S&J, Theophilakis worked at Bates London as the executive European creative director. He also worked in Singapore and then Hong Kong as the executive regional creative director.
Theophilakis began his career at Ogilvy & Mather in Oslo. He said: "I could say I'm joining JWT because of its strategic competence, strong international capabilities, insight-driven work or creative renaissance and all of it would be true. But I'm doing this because it's a challenge. I'm not a maintenance guy."