Zac Posen was raised in the SoHo neighborhood of lower Manhattan, the son of artist Stephen Posen and corporate lawyer Susan Posen. His interest in fashion design started early, and as a child he would steal yarmulkes from his grandparents' synagogue to make ball dresses for dolls. He attended Saint Ann's School, a private school in Brooklyn, and in his sophomore year interned with fashion designer Nicole Miller. At age 16 he enrolled in the pre-college program at Parsons The New School for Design. He graduated from Saint Ann's in 1999. For three years Posen was mentored by curator Richard Martin at The Costume Institute of The Metropolitan Museum of Art. At 18, he was accepted into the womenswear degree program at London's Central Saint Martins College of Art and Design at the University of the Arts London. During his tenure in London, Posen found that the environment was competitive, while at the same time students were given a wide berth for independent learning. In 2001, Posen constructed a gown entirely made from thin leather strips and dress-maker hooks and eyes that was displayed by the Victoria and Albert Museum and featured in their "Curvaceous" exhibition.
In 2000, Posen received a big break after a dress he made for Naomi Campbell changed hands several times among several fashion insiders, including actress Paz de la Huerta. Through Interview magazine Editor-in-Chief Ingrid Sischy, Posen met his future publicist and event producer Ed Filipowski, of KCD who offered to represent him for free.
Upon returning to New York in 2001, Posen set up an atelier in his parent’s living room and in October of the same year he was chosen to present a capsule collection as part of GenArt's Fresh Faces in Fashion New York 2001.
After his first runway show in 2001, Posen was courted by fashion titans Yves Carcelle (President of LVMH Fashion Group), Sidney Toledano (Chief Executive Officer and Director of Christian Dior S.A.), and Domenico De Sole (President and Chief Executive Officer of Gucci Group NV). Following the success of this presentation, Posen established his design studio in Tribeca.
Zac Posen continues to receive awards and accolades, most notably the Council of Fashion Designers of America's 2004 Swarovski's Perry Ellis Award for Womenswear. Posen’s strong, feminine aesthetic has become a favorite of style leaders including Natalie Portman, Rihanna, Kate Winslet, Claire Danes, Cameron Diaz, Jennifer Lopez, Mischa Barton, Beyoncé Knowles, and many more.
In 2004, high-end urban fashion brand Sean John made an investment deal with Posen.
In April 2008, Posen's capsule collection created in collaboration with Target, Zac Posen for Target, was released in 75 stores in Australia. A preview of the collection was shown during March's L'Oreal Melbourne Fashion Festival as one of the fashion week's headline catwalks.