Born in Zonguldak, Black Sea, Ergün begun his professional career with Gençlerbirliği SK. After that, in the 1994 summer, he signed with top division giants Galatasaray SK, being one of the team's most influential players in the following decade (in a variety of positions, including left back), and playing a very important part in the side's exploits from 1999–2001, which brought a total of six titles – two domestic doubles, the UEFA Cup and the UEFA Super Cup; in the first competition's final, he scored in the penalty shootout win against Arsenal.