Born in Oslo, Norway, Holby grew up in Larchmont, New York and attended Murray Avenue School and Mamaroneck High School. Later she took courses in apparel design at Rhode Island School of Design and started working as a fabric designer for a textile company.
Known as Clotilde, Darnell became a successful model for various couture houses, including Yves St. Laurent, Ralph Lauren, Givenchy and Chanel. She also appeared in fashion and cosmetic advertising on TV and in print.
Darnell is mother to three children, through her marriage to physicist Sebastian White. They married in March 1984. In the early 90s, she retired from modeling and relocated to Larchmont, New York, where she met her current husband, James Edwin Darnell, Jr., Vincent Astor Professor Emeritus of Biology at Rockefeller University. She established a brand of women's dresses and opened Clotilde: The Dress Shop in Larchmont, NY.