Robert Duncan was born in Oakland, California in 1919. His mother died and childbirth and his father, unable to afford to keep his son, put him up for adoption. Duncan's adoptive parents were Theosophists and from a young age he was exposed to séances and had access to a library of occult writings and mythology.

In 1936 Duncan entered the University of California at Berkley, quitting in 1938 to attend the Black Mountain College before leaving for the East Coast. By 1940 he was living in Woodstock, New York and working with James Cooney on his magazine The Phoenix, during which time he met Henry Miller, who was said to admire and support his poetry.

In 1945 Duncan returned to California where he established friendships with the early San Francisco Renaissance poets Kenneth Rexroth and Jack Spicer. Re-entering Berkley around this time he became a popular figure on the artistic scene in San Francisco, writing for Cid Corman's Origin magazine and later the Black Mountain Review. It was through these connections that Duncan was able to unite Black Mountain poets, such as Robert Creeley, with the developments San Francisco Renaissance. This scene would go on to stage the famous Six Gallery reading of 1955, which became the most seminal public gathering of the Beat era.

In the 1960s Duncan's poetry began to gain international renown and he was awarded the Harriet Monroe Memorial Prize in 1961, a Guggenheim Fellowship in 1963 and the Levinson Prize from Poetry magazine in 1964. He also received three writing fellowships from the National Endowment for the Arts and the National Poetry Award in 1985.