Mary Kate Olsen

Mary-Kate Olsen was born on June 13, 1986, in Sherman Oaks, California. She is the daughter of Jarnette "Jarnie" (née Jones; b. 1954), a personal manager, and David "Dave" Olsen, a real estate developer and mortgage banker. She is two minutes younger than her twin sister Ashley Olsen. She has an older brother Trent, a younger sister (actress Elizabeth Olsen), and two half-siblings Courtney and Jake (from her father's second marriage). Olsen's parents divorced in 1996. The twins and their siblings have Norwegian ancestry while their mother is of French, German and Italian ancestry.

Olsen attended the Campbell Hall School in Los Angeles. After graduating from Campbell Hall in 2004, Olsen and her twin sister went on to attend the Gallatin School of Individualized Study at New York University. Olsen began horse riding in 1992 when she was six years old. During her time at Campbell Hall, she was a member of the school's equestrian team. After moving to New York in 2004 to attend NYU, Olsen temporarily quit riding, but she eventually started again, stating "I came back to the sport because I missed it every day that I wasn't riding. It was the hardest thing to leave and the hardest thing to come back to." In 2013, her horse Marvelous, ridden by James Anderson, won the 38th Hampton Classic Horse Show.

Olsen is a competitive equestrian. She began riding in the Hampton Classic Horse Show in Bridgehampton, New York in 2013. On September 17, 2016, she competed at The American Gold Cup in North Salem, New York. In August–September 2017, she competed at the Hampton Classic Horse Show: on August 29, Olsen and her horse Dunotaire V (born Dunotaire VI) finished in sixth place in the amateur owner jumper (1.20 meter) event; and on September 1, she and Dunotaire V finished in third place in the amateur owner jumper classic event. In September 2017, Olsen and her horse Feu d'Amour won The American Gold Cup. She also has horses named Prem' Dollar Boy and Hertog Van't Merdehof.

In 2018, Olsen and Dunotaire V ranked 7 out of 40 in a category at the Palm Beach Masters in Wellington, Florida. In May 2019, Olsen competed at the Longines Global Champions Tour in Madrid, Spain; she and her horse Naomi finished in sixth place in their event, while she and her horse Fatum were disqualified from their event due to an "error of course". On July 12, 2019, Olsen and Fatum won a 1.25m two-phase showjumping class at the Longines Global Champions Tour in Chantilly, France. In September 2021, she competed at the Longines Global Champions Tour in Rome, Italy: on September 16, she and her horse Iowa Van Het Polderhof finished in second place in the 2 Phases Special event; and on September 18, Olsen and Dunotaire V finished in third place in an Against the Clock No Jump Off event.