He was born in the Danish West Indies (now U.S. Virgin Islands). He moved with his parents to Savannah, Georgia five years later and later to Wilmington, North Carolina. He attended Fayetteville Academy in North Carolina and Yale College. He moved to New Orleans, Louisiana, where he studied law and was admitted into the bar in 1832. Benjamin soon began private practice. He then made his home downriver from New Orleans at Belle Chasse, Louisiana.
In 1842, he was elected to the lower house of Louisiana's legislature and served there until 1844. A year later he was a member of the state constitutional convention. Benjamin was selected by the state legislature to be a Senator as a Whig; he took office on March 4, 1853. He was again selected to serve the term beginning 1859, but this time as a Democrat. During the 34th through 36th Congresses he was chairman of the Committee on Private Land Claims. President Franklin Pierce offered Benjamin a nomination to the United States Supreme Court; he declined to accept. Benjamin resigned his seat due to the secession of Louisiana in 1861.
Benjamin was the Attorney General for the Provisional Confederate Government, after which he was appointed Acting Secretary of War by President Jefferson Davis and served from August to November 1861 before becoming the full Secretary of War of the Confederate States of America on November 21, 1861. Benjamin resigned in February 1862 to accept an appointment as Secretary of State, where he remained until the conclusion of the American Civil War in 1865. He was the first man of the Jewish faith to serve in the cabinet of a North American president.
Benjamin subsequently moved to England and studied British law, passing the bar there in 1866. He became a Queen's Chancellor in 1872, the only person not born in England to hold the position. The title allowed him to plead in the House of Lords, which acted as that country's court of last resort. While in England he entered into magazine and newspaper work. In 1883, he retired and moved to Paris, where he died a year later.
See also: Jefferson Davis