Barker was born in 1869 in Leicester, Vermont. Her parents both died while Barker was still young. She worked as a shorthand writer, a teacher and wrote for newspapers. Her father had been interested in the occult and Barker shared this interest, becoming a member of the Theosophical Society. She was also initiated into the Rosicrucian Order of Alpha et Omega.

Barker lived in Europe from 1910 to 1914, first in Paris and then in London. She was in London at the outbreak of World War I. In 1912, while in Paris, she felt compelled to write a passage, although she said she did not know where the words came from. She said she was "strongly impelled to take up a pencil and write." She signed the passage "X", which at first meant nothing to her. She was told that "X" was the nickname of a Los Angeles judge called David P. Hatch and then discovered that Hatch had died before she "received" the message. In 1914 she published a book of these messages called Letters from a Living Dead Man. She said that the passages were genuine messages from the dead man and Hatch's son also believed that the communications were from his father. She published two more volumes of Hatch's messages ... War Letters from the Living Dead Man (1915), and Last Letters From the Living Dead Man (1919).

Around the time of the publication of War Letters from the Living Dead Man in 1915, Barker developed an interest in psychoanalysis. By 1919 she was studying 14 hours a day. From 1928 to 1930 she lived on the French Riviera. Barker died August 31, 1954.