Johnston is a left-handed polo player. In the 1950s, he organized polo matches on his family farm in McDonald, Tennessee. He later founded the Chattanooga Polo Club, later known as the Bendabout Polo Club. He served as the long-time President of the Gulfstream Polo Club in Lake Worth, Florida. He served as Vice President of the United States Polo Association from 1979-1989, President from 1980-1984 and as Chairman from 1984-1988, and still sits on its Board as a governor-at-large. He owns the Flying H ranch in Big Horn, Wyoming, home to the Flying H Polo Club, and the Everglades Polo Club in Wellington, Florida.