Susan Warner was born on July 11, 1819, the daughter of Henry Warner, a New York attorney, and Anna Bartlett. Indulged by her wealthy parents, Susan "had no doubt of her royalty," according to her sister and biographer Anna Warner (34). Although the death of their mother in 1826 must have been traumatic for seven-year-old Susan, Henry's sister, Frances Warner, joined the household to keep house for her brother, and Henry's law practice flourished throughout Susan's childhood. The Warners lived in spacious townhouses with gardens at fashionable addresses in New York City, summering in upstate New York or in rural Brooklyn. Warner's earliest journals mention visits to Betsy Jumel and Julia Ward (later Howe), dancing school, lavish parties with "champaigne" (170) and cotillions, and evenings at concerts or the opera.
Warner's formal education was desultory, stressing genteel accomplishments. Her lessons included French, Italian, singing, dancing, and piano; she read history, theology, and mathematics following a program designed by Henry but received little supervision. Daily household tasks included sweeping the parlors and rubbing the doorknobs.
Warner's fascination with language and storytelling began in childhood. Her favorite activity was "talking stories" (102): inventing tales with her sister or cousins as her audience. She read voraciously and responded emotionally to authors such as Sir Walter Scott, Fanny Burney, Hannah More, Maria Edgeworth, Mary Sherwood, and Amelia Opie, and she rewrote stories she read to suit herself. Warner began a journal at the age of thirteen, producing descriptions, dialogue, and similar writing exercises. She rigorously corrected her own grammar and phrasing, aiming for precision, accuracy, and felicitous phrases.