He was younger son of the 11th Earl of Pembroke. Educated at Harrow and Oriel College, Oxford, he made a reputation at the Oxford Union as a speaker, and entered the House of Commons as Conservative member for a division of Wiltshire in 1832. Under Peel he held minor offices, and in 1845 was included in the cabinet as Secretary at War, and again held this office in 1852 - 1855, being responsible for the War Office during the Crimean War, and again in 1859.
It was Sidney Herbert who sent Florence Nightingale out to the Crimea, and he led the movement for War Office reform after the war, the hard work entailed causing his breakdown in health, so that in July 1861, having been created a baron, he had to resign office, and died on the 2nd of August 1861. His statue was placed in front of the War Office in Pall Mall. He was succeeded in the title by his eldest son, who later became the 13th Earl of Pembroke, and the barony is now merged in that earldom; his second son became 14th Earl. Another son, the Hon. Michael Herbert (1857-1904), was British Ambassador at Washington in succession to Lord Pauncefote.
She was important as a translator and as a patron of poetry; Sidney dedicated his longest work, the Arcadia, to her.
In it, Sidney partially nativized the key features of his Italian model, Petrarch: variation of emotion from poem to poem, with the attendant sense of an ongoing, but partly obscure, narrative; the philosophical trappings; the musings on the act of poetic creation itself.
Sidney wrote an early version during a stay at Mary Herbert's house; this version is narrated in a straightforward, sequential manner.
He was succeeded in the title by his eldest son, who later became the 13th Earl of Pembroke, and the barony is now merged in that earldom; his second son, Hon SidneyHerbert, was also a Member of Parliament who became 14th Earl.
Lord Herbert of Lea was married to Elizabeth Ashe A'Court, of the family of the barons Heytesbury; she became a Roman Catholic after his death along with their eldest daughter.