Katherine Sian Moennig was born on December 29, 1976. She grew up in Philadelphia, Pennsylvania, with her father, a violin maker, and her mother, a Broadway dancer. Katherine was born into an extended family of industry professionals. She is first cousins with Gwyneth Paltrow and Blythe Danner is her aunt. Katherine grew up surrounded by the entertainment industry and pursued acting from an early age.
Her first acting role came at age 10 in a Children's Theatre Workshop presentation of Winnie the Pooh. The experience led Katherine and a friend to write an improvised production of the story and perform it at the Free Library in Philadelphia. While in high school, Katherine kept the desire to pursue acting on the backburner until her studies were complete.
In 1994, at the age of 18, Katherine moved to New York City to make her call to acting official; she entered the American Academy of Dramatic Arts.