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.