Seattle Slew was voted American Horse of the Year in 1977, the year in which he became the tenth of thirteen horses to win the American Triple Crown. In fact, Until Justify, in 2018, Seattle Slew was the only undefeated American Triple Crown winner in history.
Seattle Slew was bred by Ben S. Castleman Sr., owned by Mickey and Karen Taylor and Jim and Sally Hill – who became known, collectively, as the “Slew Crew” – and trained, initially, by William H. “Billy” Turner Jr. He won all three starts as a juvenile, including the Grade 1 Champagne Stakes at Belmont Park, by 9¾ lengths, and was named American Champion Two-Year-Old Male Horse in 1976.
He also won the first three starts of his three-year-old campaign and, undefeated in six starts, was sent off 1/2 favourite for the Kentucky Derby. Ridden by Jean Cruguet, Seattle Slew pulled away at the top of the home straight, eventually beating Run Dusty Run by 1¾ lengths. He subsequently started 2/5 favourite for the Preakness Stakes and the Belmont Stakes, beating Iron Constitution by 1½ lengths in the former and Run Dusty Run by 4 lengths in the latter, to take his career record to nine wins from nine starts.
Later that season, Seattle Slew was shipped, somewhat controversially, to California, with connections lured by the increased prize money, of over $300,000, offered for the Swaps Stakes at Hollywood Park Racetrack. Having been unable to dominate and boxed in for most of the way, Seattle Slew couldn’t land a blow behind the winner, J.O. Tobin, and eventually finished fourth, beaten 16 lengths. Subsequent recriminations led to the sacking of Turner and his replacement with Douglas R. Peterson.
As a four-year-old, in 1978, Seattle Slew was beaten, at odds of 1/5, in the Paterson Handicap at Meadowlands Racetrack, after which Jean Cruguet lost the ride. However, under new jockey, Angel Cordero Jr., he beat Triple Crown winner Affirmed by 3 lengths in the Marlboro Cup at Belmont Park, in a time just 0.40 seconds outside the world record set by Secretariat in the same race five years earlier. At the end of his career, Seattle Slew had won 14 of his 17 races and over $1.2 million in prize money.