She was born Marianne Craig Moore in Kirkwood, Missouri, the daughter of John Milton Moore, a construction engineer and inventor, and Mary Warner. Moore had an older brother, John Warner Moore. She never met her father; before her birth his invention of a smokeless furnace failed, and he had a nervous and mental breakdown and was hospitalized in Massachusetts. Moore's mother became a housekeeper for John Riddle Warner, her father, an, affectionate, well-read Presbyterian pastor in Kirkwood, until his death in 1894. Moore's mother, always overly protective, moved with her children briefly to Pittsburgh, Pennsylvania, and then to Carlisle, Pennsylvania, where Moore attended the Metzger Institute (now part of Dickinson College) through high school. In 1905 she entered Bryn Mawr College, in Bryn Mawr, Pennsylvania; published nine poems, including "A Jelly-Fish," in its literary magazines Tipyn O'Bob and the Lantern; and majored in history, law, and politics, graduating with a B.A. in 1909. Much--perhaps too much--has been made of Moore's later casual assertion that laboratory studies in biology and histology caused her to consider studying medicine; at any rate, one result of such work was her love of intricately shaped animals and also a lifelong respect for precision in description. She also expressed a desire to become a painter. After taking secretarial courses at Carlisle Commercial College (1910-1911), she taught bookkeeping, stenography, and typing and commercial English and law at the U.S. Industrial Indian School at Carlisle with admirable success until 1915. One of her students was Jim Thorpe, the famous Native American athlete.
In the summer of 1911 Moore and her mother traveled in England, Scotland, and France, and while abroad they visited art museums in Glasgow, Oxford, London, and Paris. In 1915 Moore began to publish poems professionally. Seven poems (including "To the Soul of 'Progress," displaying her early habit of rhyming single-syllable lines, sometimes spaced apart) appeared in the Egoist, a London bimonthly edited by Hilda Doolittle (H.