Agatha Mary Clarissa Miller was born on 15 September 1890 in Torquay, England. Her father, Frederick, was an outgoing American with an independent income. Her mother, Clara, was rather shy; Agatha resembled her greatly in personality. There were two other children - Madge and Monty, both older than Agatha.
Although Madge received a formal education, Clara decided Agatha should not. She intended that Agatha be taught to read when she was eight; however, by the age of five Agatha had already taught herself to read. The rest of her education was through a mixture of tutors, part-time schooling and French finishing schools. She also trained as a singer and pianist and had it not been for her extreme shyness, she had the talent to have made this her career.
When Agatha was eleven her father died and she became even closer to her mother. Without Frederick, Clara became restless and began to travel, at times taking Agatha with her; these early trips began Agatha's lifelong love of travel.