William Page studied at Phillips Academy, Andover in 1828-29 (not the Andover Theological Seminary on the same campus, as is commonly asserted). A man of mercurial temperament, Page was lacking in religious belief in youth, but later became a Swedenborgian. He received his training in art from Samuel F. B. Morse (a Phillips Academy graduate) at the National Academy of Design, and in 1836 he became a National Academician. In the 1830s and 40s, Page was based in New York, achieving renown there as a portraitist.
Living in Rome from 1849 to 1860, he befriended Robert and Elizabeth Browning, whose portraits he painted. He was also a friend of William Wetmore Story and of James Russell Lowell, who dedicated his first collection of poems to him in 1843.
In 1873, Page became president of the National Academy of Design. His work includes a painting of Admiral David Farragut at the Battle of Mobile Bay, the Holy Family (now at the Boston Athenaeum) and The Young Merchants (now at Pennsylvania Academy of the Fine Arts in Philadelphia), as well as countless portraits, including portraits of John Quincy Adams, James Russell Lowell and William Shakespeare, based on the Becker death mask. He also wrote A New Geometrical Method of Measuring the Human Figure (1860).
He died in 1885, aged 74 on Staten Island. Although extravagantly praised as an artist from the 1830s into the 1860s, Page's reputation suffered in later life because he changed his style so frequently and, more particularly, because technical characteristics of his painting method soon caused much of his work to darken excessively.