Deumi started his career in his native country of Cameroon playing for UCB Douala and Kadji Sport Academy de Douala. In 1999 he transferred to Switzerland to play for FC Sion. Deumi spent three years at FC Sion and was impressive enough to earn a move to Swiss league rivals FC Thun. He became a crucial part of the team but missed a large part of the 2003–04 season after suffering a ruptured knee ligament. After recovering from the injury, Deumi quickly reestablished himself in the FC Thun side and was considered by many to be the best defender playing in Switzerland as FC Thun achieved a second place finish and a spot in next year's UEFA Champions League.