At the 2002 IAAF World Cross Country Championships in Lausanne he fared slightly better, finishing 7th in the junior race. At the 2003 All-Africa Games, he was fifth in men's 10,000 metres. He made his Olympic debut at the 2004 Athens Olympics and was seventh overall in the 10,000 metres final. Mosop also won the Giro Podistico di Pettinengo 9.6 km race in 2004.