Kemboi was part of the 4x400 relay team that finished fifth at the 1991 World Championships. He competed at the Olympics for first time at the 1992 Summer Olympics, taking part in 400 metres, where he reached semifinal. He was also part of the Kenyan 4x400 relay team that advanced to the final, but did not finish it. At the 1992 IAAF World Cup, he was part of the African 4 × 400 m team that won gold.
At the 1993 World Championships, he won silver part of the Kenyan 4 × 400 m relay team. Other members of the team were Samson Kitur, Kennedy Ochieng and Abednego Matilu. In the 400 metres indivivual race he was 7th. He won bronze over 400 metres at the 1993 African Championships in a race won by compatriot Kennedy Ochieng, and may have been part of the Kenyan 4 × 400 m team that won gold.
The second of his two Olympics assignment was at the 1996 Summer Olympics, when he competed only at the 4x400 metres relay race, where Kenya reached the final again, but did not start it. He won silver over 400 metres at the 1996 African Championships in Athletics. The Kenyan 4 × 400 m team won also silver.
- Men 4x400m Relay World Championship 1991 Tokyo (JPN) - Sunday 01.09
- Sports-reference.com - Simon Kemboi
- Men 4x400m Relay Olympic Games Barcelona(ESP) 1992
- IAAF profile for Simon Kemboi
- Men 4x400m Relay World Championship 1993 Stuttgart (GER) - Sunday 22.08
- Men World Championship 1993 Stuttgart (GER) - Tuesday 17.08
- gbrathletics.com: AFRICAN CHAMPIONSHIPS
- Men 4x400m Relay Olympic Games Atlanta (USA) 1996
- The Guardian September 20, 2009: Gold rush on after Jones rival falls