The ancestors of both Sedgwick's parents were English.
Around 1634 Catharine Sedgwick's mother's ancestor John Dwight came with his wife Hannah, daughter Hannah, and sons Timothy Dwight and John Dwight, from Dedham, Essex, England to North America where the town was named Dedham, Massachusetts.
From the father's side Catharine Sedgwick's ancestor Major General Robert Sedgwick who was was an English colonist arrived in 1636 in the Massachusetts Bay Colony, as part of the Great Migration.
Catharine Maria Sedgwick came from an important Federalist family in western Massachusetts. While Catharine always spoke with love and respect of her mother, Pamela Dwight Sedgwick suffered repeated periods of mental illness and does not seem to have been close to her daughter. Instead, Catharine admired her father, though he was often away for his political career, which culminated in his becoming Speaker of the House. In his absence Catharine was surrounded by her many siblings. She was particularly attached to her four brothers. Even when they had all married and become lawyers like their father, they remained the central figures in her emotional life. Single herself, she passed part of every year in the family of one or the other of her brothers, and was a favorite aunt to many children. It was her brothers who encouraged her to write. Together they worked to sustain her often failing self-confidence, and assisted her practically with contracts and reviews.