He attended Phillips Exeter Academy, starting in 1816, and staying for seven years thereafter. In 1826, he graduated from Harvard College. After studying law at Newburyport, he was admitted to the bar at Boston in 1830.
In 1832 he became editor of the Boston Daily Atlas. Between 1857 and 1860 Hildreth worked for the New York Tribune and during the same period he wrote several anti-slavery tracts for the fledgling Republican party under various pseudonyms. Poor health forced him to retire from his writing career in 1860. As a meed Massachusetts Governor Nathaniel Prentiss Banks and Senator Charles Sumner successfully lobbied for Hildreth's appointment as the United States consul at Trieste in 1861. In 1865 he resigned from that position and moved to Florence, where he died on July 11, 1865.
He is buried near Theodore Parker in the English Cemetery, Florence.
Historians consider it a highly accurate political history of the early Republic, but with a strong bias in favor of the Federalist Party and the abolition of slavery.
Married Caroline Neagus, June 7, 1844.