Margulies made her feature film debut in Steven Seagal's action film "Out for Justice" (1991), playing a prostitute. In 1994, Margulies was cast in the pilot episode of the NBC medical drama "ER" as Carol Hathaway, an emergency care nurse who attempted suicide. Her character was originally intended to die; however, the producers changed the plot and made Carol Hathaway a main cast member. At the same time Margulies had been offered an extended role on NBC's "", after a two-episode appearance earlier in the year. Margulies chose "ER" and remained on the show for six seasons until 2000. She won an Primetime Emmy Award for Outstanding Supporting Actress in a Drama Series in 1994, and continued to be nominated for this award every year during her tenure on "ER". She was the only series regular cast member to win a Primetime Emmy Award.