Klein was born to a Jewish family in The Bronx, New York, the son of Flore (née Stern) and Leo Klein.[1] His father was an immigrant from Hungary while his mother was the daughter of an immigrant from Austria and an American dentist.[1] He attended the High School of Art and Design and matriculated at, but never graduated from, New York's Fashion Institute of Technology, receiving an honorary Doctorate in 2003. He did his apprenticeship in 1962 at an oldline cloak-and-suit manufacturer, Dan Millstein,[citation needed] and spent five years designing at other New York shops. In 1968, he launched his first company with a childhood friend,[2] Barry K. Schwartz.[2][3]
Klein was one of several design leaders raised in the Jewish immigrant community in the Bronx, along with Robert Denning and Ralph Lauren. He became a protégé of Baron de Gunzburg,[3] through whose introductions he became the toast of the New York elite fashion scene even before he had his first mainstream success with the launch of his first jeans line. He was immediately recognized for his talent after his first major showing at New York Fashion Week. He was hailed as the new Yves Saint Laurent, and was noted for his clean lines.