Behennah was born in Singapore where her father was a school administrator. The youngest of three children, Michelle relocated to Sheffield, England, after her parents' divorce. Her mother Rosemary eventually remarried and now lives in France where she and her current husband manage an inn.
As a child, Behennah aspired to study medicine and become a plastic surgeon. Her mother saw her potential as a model. At the urging of her mother, 14-year-old Michelle entered a beauty contest sponsored by a shampoo company. Michelle won the contest.
She next entered the 1993 Ford Agency "Supermodel of the World" competition. At this event she was a finalist, along with now fellow model Magdalena Wrobel. Her career was now launched.
After her win, Behennah moved to Paris, where she spent seven years establishing her name in the modeling industry. While there, she appeared in magazines such as Elle US, Glamour Italy, Vogue Italy and Spain, Mademoiselle, and Cosmopolitan.
She has also modelled for designers such as Cynthia Rowley, Valentino, BCBG, Donna Karan, Emanuel Ungaro, Nina Ricci, Ralph Lauren, Salvatore Ferragamo, Chanel, and Thierry Mugler.
In 1999, Behennah made the move to yet another world fashion centre, New York, where her career flourished. Besides the fact that she has appeared in high-profile advertisements for Burberry, Esprit, Givenchy, Vanderbilt Jeans, Hewlett Packard, J.Crew, Kenzo, L'Oréal "Elnett" hairspray, Mango, Nina Ricci, Saks Fifth Avenue, Swatch watches, and Tommy Hilfiger.
Behennah has also modeled swimsuits for Sports Illustrated's Swimsuit Edition, in 1999, 2000 and 2001. In addition to her Sports Illustrated shoots, she was the object/subject of Joanne Gair's inaugural body painting works as part of the Sports Illustrated Swimsuit Issue.