Posen's training was unusually deep for a young American designer: a pre-college stint at Parsons, three years mentored by curator Richard Martin at the Met's Costume Institute, womenswear study at London's Central Saint Martins, and an apprenticeship with Rochas patternmaker Nicolas Caïto. An early leather-and-hooks gown was shown by the Victoria and Albert Museum in 2001.
His glamorous, architecturally constructed gowns became favourites of Rihanna, Kate Winslet, Beyoncé and Natalie Portman, and dressed the red carpets at the Oscars, Golden Globes and the Met Ball. Posen won the CFDA's Swarovski Perry Ellis Award for Womenswear in 2004, served as creative director of Brooks Brothers (2014–2020), and in 2024 became creative director of Gap and chief creative officer of Old Navy.