Born in N'Djamena, Chad, Koumadje first picked up a basketball when he was 15, after previously playing soccer before he lost his speed because of his growth spurts.

In Senegal, Koumadje met basketball recruiter Ibrahima N’Diaye, the brother of former NBA player Mamadou N'Diaye, who approached him to work out at the Flying Star Academy in Dakar.

[8] On January 4, 2020, Koumadje recorded a triple-double in a 111–88 win over the Long Island Nets, posting 12 points, 16 rebounds and a franchise-record 12 blocks.

[14] On February 25, 2021, Alba Berlin of the German Basketball Bundesliga (BBL) and EuroLeague announced that they had signed with Koumadje until the end of 2022–23 season.

In November 2024, Koumadje joined the Shandong Hi-Speed Kirin of the Chinese Basketball Association (CBA).