Posen was raised in the SoHo neighborhood of lower Manhattan. 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 high school in Brooklyn, and in his sophomore year interned with fashion designer Nicole Miller. 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.