Asmussen grew up in a racing family, spent a few seasons as a jockey and in a 25-year career has developed into one of the top trainers in the country. He won the Eclipse Award as the outstanding trainer in 2008 and again in 2009 with a record 622 victories. At the end of 2011 he ranked fifth in both career victories and purse money earned.
Asmussen operates a large stable of some 200 horses with divisions in New York, Chicago, Kentucky, Texas and Louisiana and is a perennial leader in victories in a season. He trained the Horse of the Year for three consecutive seasons: Curlin in 2007 and 2008 and the filly Rachel Alexandra in 2009. Both of those horses were purchased by owner Jess Jackson, who died in April 2011, and transferred to Asmussen's care.
In 2011, his many stakes winners included the unbeaten 2-year-old filly champion My Miss Aurelia. In 2009, he handled the champion male sprinter, Kodiak Kowboy. Asmussen's parents, Keith and Marilyn, are both trainers, and his older brother Cash was a top rider in Europe for many years and now is a trainer.
Asmussen trains 2014 Kentucky Derby contender Tapiture as well as 2014 Kentucky Oaks fillies Untapable and Euphrosyne.