Born a British subject (St. Croix was then a part of the British Virgin Islands), Benjamin was taken to the United States in his early youth, settling in Charleston, S.C. For two years (1825–27) he studied law at Yale University and then settled in New Orleans, La. He was admitted to the Louisiana bar in 1832, and his practice became extremely successful in the fields of commercial and insurance law. He also prospered for a time as a sugar planter, helped to organize the Illinois Central Railroad, and was elected to the Louisiana legislature in 1842. In the U.S. Senate he was noted for his proslavery speeches. After his state had seceded from the Union, he was appointed attorney general in the Confederate government (Feb. 21, 1861). Later that year he was named secretary of war by his friend President Jefferson Davis. It was charged that his mismanagement of the war office led to several major military defeats, and he resigned, but Davis promptly named him secretary of state (Feb. 7, 1862). Late in the war he enraged many white Southerners by urging that slaves be recruited into the Confederate Army and emancipated after their term of service.

At the end of the Civil War Benjamin escaped to England, where he was called to the bar (June 1866) after only five months' residence and where he achieved his greatest professional success. In 1872 he became a queen's counsel. His Treatise on the Law of Sale of Personal Property (1868) was the principal textbook on its subject for many years in England and the United States.