He replaces the long-serving Nicholas Ng'ang'a, who has retired.

The move is part of a shareholder deal reached last year.

Under it the government, which holds a 35% stake in the company, won the right to pick a Kenyan as CEO, while the other major shareholder, Vodacom, got to pick the chairman, sources told Reuters last year. Vodacom is part of Britain's Vodafone Group.

Peter Ndegwa, a Kenyan national, took over as chief executive in April, joining from Diageo, where he was the managing director of continental Europe.

Joseph, a dual American-Kenyan national, was Safaricom CEO until 2010 and has been a board member of the company since then. Ng'ang'a had been chairman since 2007.

(Reporting by Duncan Miriri; editing by John Stonestreet)