Miller went west with his family and led a picaresque early life in California among miners, gamblers, and Indians. He attended Columbia College (Eugene, Ore.) briefly in 1858–59 and was admitted to the Oregon bar in 1860. Between 1862 and 1866 he owned a pony express and a newspaper (the Eugene Democratic Register) and was a county judge in Canyon City, Ore. For the Register he wrote an article defending the Mexican brigand Joaquin Murietta, whose given name he later used as a pseudonym. His first books of poems, Specimens (1868) and Joaquin et al. (1869), attracted little attention.

In 1870 he traveled to England, where his exotic manners and flamboyant western costume made him a great favourite with the literati. Pacific Poems (1871) was privately printed there. Songs of the Sierras (1871), upon which his reputation mainly rests, was loudly acclaimed in England, while generally derided in the United States for its excessive romanticism. His other books of poetry included Songs of the Sunlands (1873), The Ship in the Desert (1875), The Baroness of New York (1877), Songs of Italy (1878), Memorie and Rime (1884), and the Complete Poetical Works (1897).

Whitmanesque in temper, his work is frequently bombastic and artificial. Because of his fondness for Byronic posturings, his autobiographical writings (e.g., Life Among the Modocs, 1873) are usually considered untrustworthy.