Kostis begun his professional career at Iraklis, where at a young age became the club's best striker. This made AEK Athens and Olympiacos battle for his signature during the summer of 1994. The officials of Olympiacos and Iraklis agreed for the transfer and on 31 May Kostis took a flight for Athens in order to sign his new contract with the red and whites. Surprisingly, he found the officials of AEK Athens waiting for him at the airport, as during the flight they made a better offer to Iraklis and acquired him^[2] for fee at time of 350 million drachmas.^[3][4]

His first season in the team, was a period of adjustment, nevertheless he was the team's first player in appearances in the league scoring 11 times. The following season alongside Batista, Tsiartas, Ketsbaia and Savevski, he formed an incredible offensive midfield line. On 28 October 1995 he scored in the derby against Olympiacos at home making the final 1–1.^[5] In 18 November of the same year, he also scored the winner in the Athenian derby against Panathinaikos.^[6] On 3 January 1996 he scored at the away win against Olympiacos for the first leg of the round of 16 for the cup.^[7] Competing during this particular season mainly on the left flank of the attack, he was again first in appearances in the league scoring 19 times, while he won the Cup with the yellow-blacks. His best season was in 1996–97, where he won the cup again, while he was again their first player in league appearances and the top scorer with 21 goals. He scored again in the league game against Olympiacos making the final 2–0 for AEK on 13 January 1997.^[8] But at the start of the next season his career was cut short when on 23 October 1997 he got seriously injured after a collision with the goalkeeper of Sturm Graz, Kazimierz Sidorczuk during the first leg of the second round of the Cup Winners' Cup.^[9] Since then he has virtually never been able to become the same player again. During his spell at AEK he won three Cups and one Super Cup.

Kostis underwent surgery, underwent treatments and in the summer 1998, while he was still injured, he signed for Anderlecht, but did not manage to play at all for the Belgian team.^[10]

On 19 July 2000, after various surgeries, he returned to AEK,^[11] but a series of new injuries prevented him from returning in his previous form. Thus, despite his efforts to return at a competitive level and the support of the administration of AEK, he eventually did not succeed. During his second spell at the club he made only seven league appearances. In the summer of 2005 he moved to Cyprus and signed for Alki Larnaca for one season. Afterwards, he moved to Ajax Cape Town, where he finished his career in the summer of 2007.^[12]

As of 11 October 2024