The move splits Reid's remit into two, with Europe chief Scott Hollingsworth now also overseeing Latin America and the Middle East. Barnes will continue to be based in Hong Kong, while Hollingsworth's base will remain in London.
The pair will report directly to FCB worldwide CEO Brendan Ryan.
Barnes said of Africa: "FCB is the largest agency on the continent from a billings point of view, with South Africa being one of the more significant markets. My mission will be to maintain our leading position there as well as continue to grow the agency here in Asia-Pacific."
He added that the decision not to replace Reid was not unusual "because most networks do not have a head of international".
Reid, credited with significantly growing FCB's business outside the United States, served as FCB head of international for the past 10 years.
He is better known as the architect of Ogilvy & Mather Asia-Pacific. His decision to retire takes effect at the end of this month.