Herbert entered the House of Commons as Conservativemember of Parliament for a division of Wiltshire in 1832. Under Robert Peel he held minor offices, and in 1845 was included in the cabinet as Secretary at War, and again held this office from 1852 to 1854, being responsible for the War Office during the Crimean War. He was yet again appointed Secretary at War in 1859, by which time the post had been combined with the office of Secretary of State for War. Herbert was a member of the Canterbury Association from 20 March 1848. He ran the Pembroke family estates, centered at Wilton House, Wiltshire, for most of his adult life. His elder half-brother, Robert Herbert, 12th Earl of Pembroke, had chosen to live in exile in Paris after a disastrous marriage in 1814 to a Sicilian princess, Ottavia Spinelli, widow of Prince Ercole Branciforte di Butera, and daughter of the Duke of Laurino, and a subsequent liaison with Alexina Gallot, which resulted in four illegitimate children. Sidney Herbert asked his friend Florence Nightingale to lead a team of nurses out to Scutari during the Crimean War, and together he and Nightingale led the movement after the war for Army Health and reform of the War Office. The hard work entailed caused a breakdown in his health, so that in July 1861, having been created a baron in the peerage of the United Kingdom, he had to resign government office. He died from Bright's disease on 2 August 1861. His statue by Foley was placed in front of the War Office in Pall Mall, London, and subsequently, following that building's demolition, placed next to A. G. Walker's statue of Florence Nightingale in Waterloo Place, adjacent to the Crimean Monument.
Family
In the early 1840s, Herbert is thought to have had an affair with the noted society beauty and author Caroline Norton, who was unable to get a divorce from an abusive husband, so that the relationship ended in 1846. In 1846 Herbert married Elizabeth, only daughter of Lt.-Gen. Charles Ashe à Court-Repington and niece of William à Court, 1st Baron Heytesbury. She was a philanthropist, author and translator, and a friend of Benjamin Disraeli, Cardinal Manning and Cardinal Vaughan. After her husband's death, Lady Herbert became an "ardent ultramontane" Roman Catholic, along with their eldest daughter, Mary. Sidney and Elizabeth Herbert lived at 49 Belgrave Square, London, and had seven children:
Sidney Herbert is buried in the churchyard at Wilton, rebuilt by his father in neo-Romanesque style, with inside the church a marble monumental effigy of him beside Elizabeth, his wife. Herbert Sound in the Antarctic and Pembroke, Ontario in Canada are named after Sidney Herbert. Mount Herbert, the highest peak on Banks Peninsula, was named by the chief surveyor of the Canterbury Association, Joseph Thomas, in 1849.