Thelma Ritter was born on February 14, 1902, in Brooklyn, New York, and she grew up in a modest household with her parents, Charles and Lucy Ritter. Her father worked as a bookkeeper and later became an office manager. From a young age, she showed a keen interest in acting, performing in school plays and local dramatic societies.

At the age of 11, she played Puck in a production of "A Midsummer Night's Dream," which marked the beginning of her journey in the world of theater.

After graduating from Manual Training High School in Park Slope, Brooklyn, she attended the American Academy of Dramatic Arts, although she initially struggled to establish a stage career. She took a hiatus from acting to raise her two children, Monica and Joe, with her husband, Joseph Moran, who was also an actor before becoming an advertising executive.

Her first professional experience came with stock theater companies in New York and New England, and she eventually made her Broadway debut in "The Shelf" in 1926.

Her film career began with a small, uncredited role in "Miracle on 34th Street" (1947), where she played a weary Christmas shopper. Her performance caught the attention of director George Seaton, who expanded her role.

She continued to work steadily in films, often portraying working-class characters with her strong New York City accent and plain appearance. Some of her notable early roles include "A Letter to Three Wives" (1949) and "All About Eve" (1950), for which she received her first Academy Award nomination.

Throughout her career, Thelma Ritter earned six Academy Award nominations for Best Supporting Actress, a record that still stands in that category. Her other notable films include "Pickup on South Street" (1953), "Rear Window" (1954), "Pillow Talk" (1959), and "Birdman of Alcatraz" (1962).

She also won a Tony Award for Best Actress in a Musical for her performance in "New Girl in Town" (1956), sharing the award with her co-star Gwen Verdon. She and her husband Joseph Moran were married in 1927, and they supported each other in their respective careers.

She passed away on February 5, 1969, in New York City.