William Herbert was the only son from the short-lived marriage of the future 5th earl of Pembroke and his first wife Penelope Bayning, the widow of Paul Bayning†, 2nd Viscount Bayning. He travelled abroad in France in 1658-9, led by such recipients of his father’s literary patronage as Sir John Denham and Sir Richard Fanshawe. He was returned for Glamorgan at the elections for the Cavalier Parliament, but it is difficult to know his activities in the Commons because of the presence there at the same time of another Lord Herbert, Henry Somerset, styled Lord Herbert of Raglan (later duke of Beaufort).
After his father’s death on 11 Dec. 1669, the newly elevated 6th earl of Pembroke never once attended the House before his death in July 1674. He was, however, reasonably diligent in ensuring that his vote was cast by proxy, first registering it on 13 Mar. 1670 with his kinsman Aubrey de Vere, 20th earl of Oxford, who had married Pembroke’s step-sister Lady Anne Bayning. On 1 Feb. 1671 a petition of some of the late earl’s creditors asking that his goods, then in the possession of the present earl, might be sold to satisfy his debts was dismissed by the House for being contrary to the privilege of Parliament.
For the following short session of spring 1673, Pembroke’s proxy in favour of the west country peer Anthony Ashley Cooper, earl of Shaftesbury, a long colleague and proxy recipient of Pembroke’s father, was registered on 3 Feb. 1673, one day before the session formally opened, and cost Pembroke £4 10s. in fees. He once again registered it with Shaftesbury on 27 Dec. 1673, well in advance of the opening of the session of Jan.-Feb. 1674.
Pembroke was unmarried, although earlier he had been linked to the heiress Elizabeth Malet, when he died at an early age in July 1674.