Koskei trained in Uganda early in his career and won a silver medal for his adopted country in the 400 metres hurdles at the 1970 British Commonwealth Games.

[1] His appearance at the 1972 Summer Olympics came after he switched allegiances back to his native Kenya.

[2] He competed in the 400 metres hurdles where he was eliminated following his first heat, which he finished in fourth position.

At the 1974 British Commonwealth Games he was also a member of Kenya's gold medal winning 4 × 400 metres relay team.

He won another gold medal with the 4 × 400 metres relay team at the 1978 Commonwealth Games.