William John Hennessy was an Irish-American artist.

William John Hennessy (originally Ó hAonghusa) was born in Thomastown, County Kilkenny in 1839. His father, John Hennessy, was forced to leave Ireland in 1848 as a result of his involvement in the Young Ireland movement. He landed in Canada and settled in New York City. William, his mother Catherine, and brother joined their father there in 1849. He gained admittance to the National Academy of Design in 1854 and exhibitioned his first works there.

Hennessy was very skilled in wood engraving and was hired to illustrate the works of renowned poets, including that of Tennyson, Longfellow and Whittier. As an American he became the co-founder of the Artists' Fund Society, and an honorary member of the American Society of Painters in Watercolours. In 1870 he moved to London where he became a member of the Royal Institute of Oil Painters in 1902. Between 1879 and 1907 the Royal Hibernian Academy displayed eight of his paintings.

He was married around 1868 to Charlotte Mather (1842-1940), from New Haven, Connecticut, a descendant of the old New England Mather family, and had by her four children.