Dana withdrew from Harvard College when measles weakened his eyesight, and he shipped to California as a sailor in August 1834 to regain his health. After voyaging among California's ports and gathering hides ashore, he rounded Cape Horn, returned home in 1836, and reentered Harvard. His travel experiences cured him physically and evoked his sympathy for the oppressed.

In 1840, the year of his admission to the bar, he published Two Years Before the Mast, a personal narrative presenting “the life of a common sailor at sea as it really is” and showing the abuses endured by his fellow sailors. The book was immediately popular, and its realistic descriptions made it an American classic. In 1841 he published The Seaman's Friend (also published as The Seaman's Manual), which became known as an authoritative guide to the legal rights and duties of seamen. Against vigorous opposition in Boston, Dana gave free legal aid to blacks captured under the Fugitive Slave Law. In 1863, while serving as U.S. attorney for Massachusetts (1861–66), he won before the U.S. Supreme Court the case of the Amy Warwick, securing the right of the Union to blockade southern ports without giving the Confederate states an international status as belligerents.

His scholarly edition of Henry Wheaton's Elements of International Law (1866) precipitated a lawsuit by an earlier editor. The charges of plagiarism that resulted from the suit contributed to Dana's defeat in a congressional election (1868) and caused the Senate to refuse his confirmation when President Ulysses S. Grant named him minister to Great Britain (1876). Among Dana's other works are To Cuba and Back (1859), Speeches in Stirring Times (1910), and An Autobiographical Sketch (1953).