Zachary E. Posen was born and raised in a Jewish family[2] in New York City, residing in the SoHo neighborhood of lower Manhattan.

[5] 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.

In 2001, Posen constructed a gown entirely made from thin leather strips and dressmaker hooks and eyes[10] that was displayed by the Victoria and Albert Museum and featured in their "Curvaceous" exhibition.

[11] In 2000, Posen received a prominent career opportunity after a dress he made for Naomi Campbell changed hands several times among several fashion insiders, including actress Paz de la Huerta.

[12] 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.

[12] 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.

[15] Posen has become a favorite of Natalie Portman, Rihanna, Amanda Seyfried, Kate Winslet, Claire Danes, Cameron Diaz, Jennifer Lopez, Mischa Barton, Beyoncé Knowles and others.

[24] Also in 2014, he was named Creative Director for Brooks Brothers, tapped to modernize their signature women's clothing and accessories line.

[30] The closure occurred soon after it was announced on August 6, 2019, that Barneys New York, a major client of Posen's company, had filed for bankruptcy.