Mensah made his debut for Sunderland on 12 September 2009, coming on as a 77th minute substitute against Hull City in a 4–1 win. He then made his full debut in the League Cup match, winning 2–0 over Birmingham City on 22 September and even managed to put the ball in the net, although it was disallowed for a foul on "Blues" goalkeeper Maik Taylor. However, Mensah's start to his Sunderland career has been hampered with the constant reoccurrence of a calf injury. It was not until on 21 November 2009 when he returned to the starting line–up, helping Sunderland win 1–0 win against Arsenal. However, his return was short–lived when Mensah was sidelined with another calf injury. But he returned from injury, coming on as a 82nd minutes substitute, in a 2–0 loss against Aston Villa on 15 December 2009. Four days later on 19 December 2009, Mensah scored his first Sunderland goal, in a 4–3 loss against Manchester City. So far Notwithstanding, teammate Michael Turner described Mensah as the best defender he has ever played with. Mensah became a favourite amongst the Sunderland faithful, but his injury problems meant he would struggle to play much in the upcoming season. Despite this, Mensah continued to remain involved in the starting eleven for the side throughout the 2009–10 season. In his first season at Sunderland, he went on to make seventeen appearances and scoring once in all competitions.