Born in Obuasi, Ashanti, Mensah is a product of the scouting system in Ghana that identifies talented youngsters long before they enter any formal structures and exports them to potential fame and fortune in Europe. A centre-half by choice, he is also adept at right-back and has played in both positions for his national side, also captaining the team on occasions. His physique has earned him the moniker the "Rock of Gibraltar", and Mensah was considered a certainty to be named to Ghana's squad for the 2006 FIFA World Cup. He spent the 2009–10 and 2010–11 seasons on loan at English club Sunderland in the Premier League.