He was the first athlete to run a half marathon in less than one hour by running 59:47 in Milan on 3 April 1993. His world record was broken five years later by fellow Kenyan Paul Tergat.
At the Chicago Marathon in 1999, Tanui helped spur Khalid Khannouchi to a new world record, eventually finishing 2nd in 2:06:16, which was a Kenyan national record and the third fastest marathon in history at that point. 
He was involved in a serious car accident near Nakuru in February 2010 suffering major injuries to his leg and chest. David Lelei, another former runner, was driving Tanui's car and died in the accident