1917: two-year-old season

On June 30, 1917, at Latonia Race Track in Covington, Kentucky, Exterminator made his debut in a six-furlong maiden race that he won by three lengths. Sent to race in Windsor, Ontario, Canada, he suffered a muscle sprain and Milam gave him time off to grow into his size, which by now was 17 hands. Still, he had earned $1,500 and a potential nomination to the Kentucky Derby.^[4]

1918: Three-year-old season

Before Exterminator could begin his third season, Milam sold him to Willis Sharpe Kilmer for $9,000 and a pair of fillies, quite a bit of money for the times, especially as Kilmer had only authorized his future U.S. Hall of Fame trainer, Henry McDaniel, to pay about $700 for a "workhorse." Kilmer bought Exterminator to help his prized colt Sun Briar in his workouts. (Kilmer had purchased his colt at the same sale Milam bought his, but for $5,000.)

Kilmer did not think much of his new purchase; he called him "that truck horse" or "the goat". In workouts, Exterminator was supposed to stay behind Sun Briar merely to urge him on to greater effort, but he ran easily beside the other horse unless held back. Sun Briar had topped his juvenile division, winning five of nine starts and being named U.S. Two-Yr-Old Champion Colt. McDaniel was impressed by Exterminator and considered him the most intelligent thoroughbred he had ever known.

As the Derby approached, Sun Briar developed ringbone and Kilmer suddenly had no horse for the race. McDaniel urged him to enter Exterminator, but Kilmer would not hear of running "that goat" in his colors. It took Colonel Matt Winn, president of Churchill Downs, to convince him. Winn had seen the colt's workouts and was very impressed.

The morning of the race, it poured with rain, and the track was deep in mud. Exterminator had not raced since age two, and none of his races could be considered a suitable prep for the prestigious Kentucky Derby. Ridden by a disappointed Willie Knapp, who had expected to be up on Sun Briar, Exterminator went off at odds of 30–1 to the heavily favored War Cloud. Exterminator raced at the back until the field turned for home, when he launched his bid. Nearing the wire, he passed Escoba and won the Derby by a length.^[5]

Later career

When Man o' War was three, Kilmer tried to arrange a match race between him and Exterminator. Man o' War's owner, Samuel Riddle, seemed to agree, but the race never took place.

As a gelding, Exterminator competed in 99 races, winning 50 while finishing second and third 17 times each. His lifetime earnings amounted to $252,996. Beaten in the Brooklyn Handicap by Grey Lag once, Exterminator got better with age and later defeated Grey Lag in the same race. Following the retirement of trainer Henry McDaniel, in 1921 Willie Knapp took over as trainer of Exterminator and won five stakes races with the horse.^[6]

Exterminator made the last start of his career on June 21, 1924, at Dorval Park in Montreal, Canada where he ran third to Albert Bostwick Jr.'s Spot Cash in the Queen's Hotel Handicap.^[7]

100-race start myth

Found frequently and in long-time error is the assertion that Exterminator started in 100 races. Per the research and subsequent book Exterminator (#18 in the Thoroughbred Legends series), author Eva Jolene Boyd reviewed all records of his starts and the record keeping by the Daily Racing Form and found evidence that he only had 99 official racing starts.

In addition, the local paper in Binghamton, New York, (where Exterminator spent his final years) noted upon his death in their front-page coverage that he had only 99 lifetime starts.^[8]

The 100th "start" was an exhibition run by Exterminator alone at Hawthorne Race Course in Cicero, Illinois, in September 1922. It was not for purse money, and none of his win, place or show finishes is affected by this walkover effort in a public workout.