Karen Lende O'Connor grew up in Bolton, Massachusetts and got her first horse, Midnight, for her 11th birthday, then began eventing through Pony Club. She later owned a Connemara named Erin's Shamrock, who took her to her first Three-Day Event at Radnor. The pair eventually moved up to the Advanced level. Karen began competing internationally in 1979 when she was 21 years old. In 1993, Karen married fellow equestrian, David O'Connor, and together they formed the famous O'Connor Event Team.

Karen has been named U.S. Female Equestrian Athlete of the Year ten times, and in 1993 was the number one ranked lady rider in the world. She earned a team silver medal at the Atlanta Olympics, riding Biko, and the team bronze medal at the Sydney Olympics aboard Prince Panache. At the Pan American Games at Fair Hill, Karen and Joker's Wild earned the individual silver medal. And in 2007, Karen and the pony Theodore O'Connor won the individual gold medal in addition to leading the U.S. to the team gold medal at the Pan Am Games in Brazil. Karen again represented the U.S. in the 2008 Olympic Games aboard Mandiba. She has won the Rolex Kentucky Three Day Event three times, in 1991, 1997 and 1999.