His elaborate article on the character and habits of Prescott, written for the New York Tribune just after the historian's death in 1859, was republished in a memorial volume issued by the Massachusetts Historical Society.

Carter joined the Free Soil Party in 1848, and in 1850 wrote for the Boston Atlas a series of articles in reply to Francis Bowen's attack on the Hungarian revolutionists.

From 1851 to 1852 he edited, at first as assistant of John G. Palfrey and afterward alone, the Boston Commonwealth, the chief exponent of the free soilers.

A short platform drawn up by him was adopted, together with the name "Republican," and on his motion a committee of six was appointed to organize the new party, John A. Andrew being made its chairman.

His next work was with George Ripley and Charles A. Dana on the first edition of the American Cyclopædia (1859 to 1863), in which many important articles were from his pen, including "Egypt," "Hindostan," "Mormons," and the history of the United States.

In January 1864, he was appointed private secretary of the treasury agent whose headquarters were at Beaufort, South Carolina; and from July of that year until October 1869, he edited the Rochester, New York, Democrat, doing such work for it as was seldom done on any but metropolitan journals.

When news came of the assassination of President Lincoln, he wrote, without consulting any book or memoranda, an article giving a brief but circumstantial account, with dates, of every celebrated case of regicide.