May 19, the former American eventing rider died of a heart attack, he represented his country at the European Championships in 1967.
He also became an Olympic back up at the Mexico City Olympics. He was introduced to the US team at the age of 16 and went on to have a successful eventing career from then on. Mason also organized the World Cup eventing in America and had his own stable where he also trained horses. He was 72 years old.
Source: Horse and Hound