On 26 April 2007, it was reported in the French media that Mensah was being seriously scouted by Lyon with a view to making a move for him in that summer's transfer window. On 18 June 2008, new Lyon head coach Claude Puel made it his priority to sign Mensah to replace centre-back Sébastien Squillaci, who departed to Sevilla in La Liga. Negotiations continued for the following four weeks, and on 15 July 2008, Lyon's special adviser Bernard Lacombe contacted Mensah and negotiated a five-year contract with him. However, Rennes dismissed Lyon's initial interest. Lyon club owner Jean-Michel Aulas offered the Breton club only €6 million, a figure that did not appease Rennes general manager Pierre Dréossi. But the French media said Mensah will sign for Lyon in an €8 million transfer. On 21 July 2008, Mensah finally signed a five-year contract with Lyon in an €8.4 million transfer. He was presented to the media at 20:00 that day. During the press conference, Aulas stated Mensah was recommended to them by ex-Lyon midfielder Michael Essien. Mensah took the number 15 shirt, and became the third Ghanaian player to ever play for Lyon, after Abedi Pele and Essien.