Missy Clark was born into the equestrian world as the daughter of locally renowned rider Doris Clark. After learning to ride under the guidance of her mother in the far southeast Buffalo suburb of East Aurora, New York, she trained under acclaimed riders such as Chuck Graham and Rodney Jenkins. In addition to her studies and practice as a horseback rider, Missy Clark earned a bachelor of science in environmental science from New England College.

Partnering with her mother, she owned and operated her first horseback riding organization under the name Fox Run. Missy Clark subsequently went into business for herself as the owner and head trainer of North Run. Irish equestrian rider John Brennan joined her at this Warren, Vermont-based training organization in 1999, and the couple married in the 2000s. Under their leadership, North Run has trained prominent riders from the US, Ireland, Canada, and Argentina, including America’s Ava Stearns who won the 2019 ASPCA Maclay National Championship.