He graduated at the Harvard Law School in 1832, and in 1833 he was admitted to the bar in Boston, where he entered into partnership with Charles Sumner, and developed an extensive legal practice.

Beginning in 1837, Hillard rented rooms to Nathaniel Hawthorne, who had recently taken a job at the customhouse in Boston.

With George Ripley, he edited the Christian Register,[6] a Unitarian weekly, beginning in 1833; in 1834, in association with Sumner,[6] he became editor of The American Jurist (1829–1843), a legal journal to which Sumner, Simon Greenleaf and Theron Metcalf contributed; and from 1856 to 1861 he was an associate editor of the Boston Courier.

"He is credited with having instilled a love of good literature, and a knowledge of the best English writers to generations of Americans".

[13] In addition to his oratorical contributions in meetings of the Massachusetts legislature, he gave the July 4 oration in Boston in 1835; a lecture on "Public Instruction in Prussia" (1836); a speech on "The Relation of the Poet to his Age," at Harvard (1843); he spoke on “Dangers and Duties of the Mercantile Profession” to the Mercantile Library Association (1850); he spoke before the New York New England Society (1851); and he delivered a eulogy on Daniel Webster in 1852.