Robert Lee Frost was born on March 26, 1874, in San Francisco, California, US, to William Prescott Frost, Jr. and his wife, Isabelle Moodie. His father, a journalist, was the descendant of an English immigrant, while his mother was a Scottish immigrant. Robert had a younger sister, Jeanie.

Robert Frost's personal life was plagued by grief and loss. In 1885 when he was 11, his father died of tuberculosis, leaving the family with just eight dollars. Frost's mother died of cancer in 1900. In 1920, he had to commit his younger sister Jeanie to a mental hospital, where she died nine years later.