A fourth-generation horseman, he trained Quarter Horses as a teenager before becoming a licensed veterinarian with a racetrack practice in the Pacific Northwest. Harrington's wife, Patricia, trained at the tracks where he was a vet, they married and join forces. After 28 years as a vet, he moved to training full-time in 1993 and is based at Hollywood Park. Harrington, who usually wears a white cowboy hat, has had success with young horses, especially Swiss Yodeler, owned by longtime client Heinz Steinman. Purchased for $60,000, the colt won five graded 2-year-old stakes in 1996 and has gone on to be a top stallion in California. One of his sons, Thor's Echo, won the 2006 Breeders' Cup Sprint. Many of the horses in Harrington's barn are sired by Swiss Yodeler.