Henry Dixon Cowell (/ˈkaʊəl/; March 11, 1897 – December 10, 1965) was an American composer, writer, pianist, publisher, teacher and the husband of Sidney Robertson Cowell. Earning a reputation as an extremely controversial performer and eccentric composer, Cowell became a leading figure of American avant-garde music for the first half of the 20th century — his writings and music serving as a great influence to similar artists at the time, including Lou Harrison, George Antheil, and John Cage, among others. He is considered one of America's most important and influential composers.

Cowell was mostly self-taught and developed a unique musical language, often blending folk melodies, dissonant counterpoint, unconventional orchestration, and themes of Irish paganism. He was an early proponent and innovator of many modernist compositional techniques and sensibilities, many for the piano, including the string piano, prepared piano, tone clusters, and graphic notation. The Tides of Manaunaun, originally a theatrical prelude, is the best-known and most widely-performed of Cowell's tone cluster pieces for piano.

Cowell was born on March 11, 1897, in rural Menlo Park, California, a suburb of San Francisco. His father, Henry Blackwood "Harry" Cowell, was a romantic poet and recent immigrant from County Clare, Ireland. His mother, Clara "Clarissa" Cowell (née Dixon), was a political activist, author, and native of the American Plains, who was 46 when she gave birth to Henry in addition to being over ten years older than her husband. Clarissa's ancestry was similarly Scotch and Irish, although her paternal lineage had been in America for centuries, with figures including astronomer Jeremiah Dixon, one of the surveyors behind the American Mason–Dixon line. After meeting for the first time, the two quickly wed and undertook bohemian lifestyles, residing in a small, crude cottage (later demolished in 1936) Harry had built on the outskirts of the city — where Henry would eventually be born. It was in his first few years that Henry had his first exposures to music.

His parents often sang to him the folk songs of their native homelands, and he was soon able to recite them before he learned to speak. During occasional visits to downtown San Francisco, he also recalled hearing the traditional music of Indonesia, China, Japan, and others. The family was gifted small instruments by friends and neighbors, including a mandolin harp and a quarter-size violin, the latter of which the young Henry took an interest in, making it his instrument of choice for a few years. His mother eventually decided to stop both the private lessons and his public school career after Cowell had severe bouts of Sydenham's chorea and scarlet fever — from which he eventually recovered.

Due to an ongoing affair between Harry and a French mistress, the Cowells amicably divorced in 1903, by which time Henry was 5. He was thereafter raised in Chinatown by his mother, who imbued him with her strong anarchist and feminist beliefs. It was during this time he exhibited a strong defiance of gender stereotypes — he refused to have his hair cut, often wore women's clothing and adored the color pink while preferring to be called "Mrs. Jones". He also had further music exposures when engaging with his new Asian-American friends and their families in the neighborhood. After the 1906 San Francisco Earthquake, much of the Cowells' possessions and memorabilia were destroyed in the ensuing fire, after which Henry and his mother fled the state of California. With no permanent place to live, Henry resided with his mother's family and friends around the American Plains and Midwest, later in New York City. School teachers of this time often took note of his "musical genius" and eccentric personality that was hindered by "extreme poverty". Lewis Terman, an eventual pioneer of the IQ test, met with the young Henry during the family's brief stay in rural Iowa. He would posit that Cowell had, "language almost literary. No college professor of English could have improved upon it. And it was so natural. His conversation breathes intelligence. I had the feeling that no unschooled boy who was not a genius of the first order could speak thus" and, "Although the IQ is satisfactory, it is matched by scores of others. [...] But there is only one Henry." Clarissa's career as a progressive feminist writer did not earn her much money, and by the time they eventually returned to San Francisco, she had become terminally ill with breast cancer. They found their home destroyed from the prior earthquake, and looted by vandals after standing unoccupied for so long. Neighbors housed the two as the then thirteen-year-old Henry restored it. In order to keep them financially afloat, he took up small jobs such as picking and selling flower bulbs at the Menlo Park Train Station, janitorial work, farming, and cleaning a neighbor's chicken houses.

While receiving no formal musical education (and little schooling of any kind beyond his mother's home tutelage), he began to compose short classical pieces in his mid-teens. Cowell saved what money he could from odd jobs, and at the age of fifteen, purchased a used upright piano for $60 ($1,772 in 2022). The piano significantly aided his compositional output — by 1914, he had written over 100 pieces, including his first surviving piece for solo piano, the repetitive Anger Dance (originally Mad Dance). He would begin experimenting in earnest, often by slamming the keyboard with all his strength, and rolling his mother's darning egg across the strings. In the same year, at the age of 17, Cowell enrolled at the University of California, Berkeley, studying composition with renowned American musicologist and composer Charles Seeger. Seeger later made note of their, "concurrent but entirely separate pursuit[s] of free composition and academic disciplines." After showing Seeger the drafts of his music, he encouraged Cowell to write about the methods and theory behind his tone clusters, which later became the draft for his book New Musical Resources.

Still a teenager, Cowell wrote the piano piece Dynamic Motion (1916), his first important work to explore the possibilities of the tone cluster (listen^ⓘ). It requires the performer to use both forearms to play massive secundal chords and calls for keys to be held down without sounding to extend its dissonant cluster overtones via sympathetic resonance. After two years at Berkeley, Seeger recommended that Cowell study at the Institute of Musical Art (later the Juilliard School of Music) in New York City. Cowell only studied there for three months (October 1916 to January 1917) before dropping out, believing the musical atmosphere was too stifling and uninspiring. It was in New York, however, where he met fellow modernist piano composer Leo Ornstein. The two would collaborate in later decades.

In February 1917, Cowell enlisted in the army to avoid being drafted in World War I and seeing direct military combat. He served in the ambulance training facility at Camp Crane, Allentown, Pennsylvania, where he had a short stint as the assistant band director for a few months. In October 1918, Cowell was transferred to Fort Ontario in Oswego, New York. He was transferred just before an outbreak of the Spanish flu killed thirteen men at Camp Crane.