In 1993, Rubenstein became a fashion assistant at "Cosmopolitan" and five years later was made the senior fashion editor. This led to Hearst Magazines president Cathleen Black asking Rubenstein to come up with a concept for a new magazine. Forty-eight hours later Rubenstein presented the idea of "CosmoGIRL!" and was offered the position of editor-in-chief. This made Rubenstein, who was 26 years old at the time, the youngest editor-in-chief in Hearst Magazine's 100-year history. Rubenstein went on to make "CosmoGIRL!" a success with a circulation of 1.25 million readers.