Taking over as the chief executive, and starting in September, he will be responsible for reinventing Modem as a digital specialist.
McCarthy will work closely with Pablo Marques, the executive creative director who was hired in June from Unit 9, and report to Nigel Jones, the chairman and chief executive of Publicis Group UK.
The pair previously worked together at DraftFCB, when McCarthy was the managing director and Jones the chief executive. McCarthy stepped into Jones' role after he left to join Publicis in May 2008.
Sebastian Dreyfuss, the current Modem CEO who was promoted into the role last September, will be moved to an as-yet unspecified job within the Publicis network.
Jones said: "Enda is a rare talent. He understands the digital world but he also understands what clients want from a digital agency. He's also a dynamic manager and has the skill and experience we need to push Modem to the next level."
McCarthy began his career as a graduate at Publicis in 1992, before working at agencies including CDP, Euro RSCG Wnek Gosper, JWT London and Nitro, where he became the general manager in 2005.
In 2006, he moved to DraftFCB, before taking over from Russell Marsh, the former managing director at Agency.com, in 2008.