In January 2008, Mensah was called up to the Ghana squad for the Africa Cup of Nations. Prior to the start to the tournament, he succeeded Stephen Appiah as captain. He played his first match of the tournament, starting game, in a 2–1 win against Guinea on 21 January 2008. Mensah then helped the side progress through to the knockout stage after keeping two clean sheets against Nambia and Morocco. On 3 February 2008, Mensah was sent off in a 2008 Africa Cup of Nations quarter-final match against Nigeria for a professional foul against Peter Odemwingie, as Ghana nonetheless won the match 2–1 with goals from Michael Essien and a later winner by Junior Agogo. He returned to the starting line–up against Ivory Coast in the third-place play-off, helping Ghana win 4–2.
