Biography
Sidney, an only child, was born Sophia Kosow in The Bronx, New York to Rebecca, a Romanian Jew, and Victor Kosow, a Russian Jewish immigrant. Sidney became an actress at the age of 15 as a way of overcoming shyness, using her stepfather's surname as her professional surname. As a student of the Theater Guild's School for Acting, Sidney appeared in several of their productions during the 1920s and earned praise from theater critics. In 1926, she was seen by a Hollywood talent scout and made her first film appearance later that year.

Career
Sidney was married three times, including to actor and acting teacher Luther Adler from 1938 until 1947, by whom she had her only child, a son, Jacob, who predeceased her. She died from throat cancer in New York City at the age of 88, after a career of more than 70 years.
Sidney has a star on the Hollywood Walk of Fame for her contribution to Motion Pictures at 6245 Hollywood Boulevard.
During the filming of Summer Wishes, Winter Dreams, costar Joanne Woodward remarked how she and her husband, Paul Newman, had a difficult time remembering their anniversary date. Later, Sidney surprised Woodward with a gift of a handmade pillow with the inscription "Paul and Joanne" and their anniversary date.