He considered standing for Monmouthshire at the 1784 election, where one of the sitting Whig members, John Hanbury, was in poor health, but found on consulting Hanbury that the latter intended to stand again. Nevill was also nominated, without his knowledge, as the candidate for Seaford on the Treasury interest and was returned there after a contest, together with Sir Peter Parker, on 30 March 1784. The losing candidates, Lewis Watson and Thomas Alves, were supported by the Pelham family, led by the Duke of Newcastle. Watson petitioned against the election, on the grounds that the bailiff had failed to give four days' notice of the election, and it was declared void on 21 March 1785. In the meantime, Nevill's father was created Earl of Abergavenny on 17 May; as his heir apparent, Henry received the courtesy title of "Viscount Nevill". Hanbury had died abroad in France on 6 April, leaving a vacancy in Monmouthshire. Hanbury's first cousin, John Hanbury Williams and Nevill both offered themselves as candidates, but Nevill obtained the support of the Henry Somerset, 5th Duke of Beaufort, Lord Lieutenant of Monmouthshire, and the Morgan family of Tredegar, and Hanbury Williams declined to contest it. Once the field was clear, he took the Stewardship of the Chiltern Hundreds to vacate his seat at Seaford and was returned uncontested for Monmouthshire. While he generally followed the lead of his father-in-law, John Robinson in supporting Pitt's administration, he is known to have broken with them on some issues, such as the scrutiny of the Westminster election of 1784. He was not to sit long in the Commons, as he succeeded his father in the Earldom on 9 April 1785. Abergavenny also served as Recorder of Harwich. Around 1790, he rebuilt the family's old house at Eridge and made it his principal seat. In 1803 or 1805, having moved to Eridge Castle, he sold his estate at Kidbrooke, near East Grinstead, to Charles Abbot. On 23 May 1814, Abergavenny was created a Knight of the Thistle. He died on 27 March 1843 at Eridge Castle, and was buried on 4 April 1843 at East Grinstead. His two elder sons having predeceased him, without children, he was succeeded by his third son John.
