[3] His father, Keith, is a retired jockey and his mother Marilyn is a trainer who became the first woman to win a major quarter horse race with Vespero in the 1978 Kansas Futurity.

The first was Stonestreet-owned Curlin,[9] who finished third in the 2007 Kentucky Derby, but then defeated Derby-winner Street Sense to win the 2007 Preakness Stakes.

[10] Curlin would win more Grade and Group 1 races in 2008, including the Dubai World Cup,[12] Stephen Foster Handicap[13] and Woodward Stakes.

She would beat colts again in the Haskell Invitational and win over older males in the Woodward Stakes en route to American Horse of the Year honors for 2009.

[15] The owners of Creator selected Asmussen to train the horse because they were impressed by his handling of other offspring by leading sire Tapit.

In 2004, he set a single-season record for wins by a trainer with 555, surpassing the previous standard of 496 held since 1976 by Jack Van Berg.

Asmussen gave credit to his assistant trainers Scott Blasi, Darren Fleming and Toby Sheets, who allow him to maintain divisions in Arkansas, Louisiana, New York, and Texas.

[5][3] Asmussen's nomination into American thoroughbred racing's Hall of Fame was removed from the agenda in 2014 because of allegations by PETA he had committed cruelty to animals.

In the interview he denied all accusations, noted that nothing in the PETA video alleged a rule violation or illegal medication use, and stated that assistant Scott Blasi had been fired due to his "disrespectful" comments about Ahmed Zayat.

[30] On May 22, 2014, Blood-Horse magazine reported they had obtained copies of the PETA complaint to the Kentucky Horse Racing Commission and the 22-minute video that was submitted to investigators through a Freedom of Information Act request.

However, it was also reported that the earlier, nine-minute video titled "Horse Racing Exposed: Drugs and Death" that PETA released on its web site had been heavily edited.

Blood-Horse noted in particular that PETA had moved audio segments, especially of Blasi's outbursts of profanity, to run with completely unrelated video clips.

[31] The Kentucky Horse Racing Commission cleared Asmussen and Blasi on January 15, 2015, based on the results of its investigation into PETA's allegations.

"[30] The New York State Gaming Commission reviewed seven hours of video provided by PETA in addition to other investigation, and announced its findings on Asmussen in a 176-page report released on November 23, 2015.

Of 14 specific allegations made by PETA, four minor infractions were sustained, three based on illegal use of a synthetic hormone, thyroxine as a feed supplement, and one a paperwork violation—resulting in a $10,000 fine.