Mettomo made his name in France, playing for St. Étienne. In September 2001, he signed for Manchester City of the English First Division for a £1.5 million fee. His Manchester City debut came as a substitute in a 6–0 League Cup defeat of Birmingham City. He played 23 league games that season, scoring once against Bradford, helping Manchester City to win the 2001–02 First Division Championship. The following season, Mettomo rarely featured in the first team due to the signing of Sylvain Distin. He made only four Premier League appearances, and in the close season signed for Kaiserslautern of the Bundesliga.
Mettomo went on trial with Norwich City in September 2007, but the club decided not to offer Mettomo a contract due to concerns over his fitness. Later that month, Mettomo signed a short term deal at English Championship side Southampton that would run until the end of the 2007–08 season; this contract was cancelled in January, however, due to Mettomo's inability to break into the first team.