On 24 May 1790 Howard was commissioned a captain in the North Battalion, Gloucestershire Militia. He first entered Parliament the same year, being returned for Arundel in Sussex, under the patronage of the Duke of Norfolk of Arundel Castle, and Steyning. His election for Steyning was overturned on petition in 1791, but he represented Arundel until 1795. He was then elected for Gloucester, which seat he held until 1818. In 1812 on the death of his uncle Sir Francis Molyneux, 7th Baronet, Howard inherited the Molyneux estates of Teversal and Wellow and under the terms of the bequest adopted the surname Molyneux-Howard. In 1815 his elder brother Bernard Howard succeeded a distant cousin as 12th Duke of Norfolk. Because Bernard was a Roman catholicrecusant, he was obliged to appoint a deputy to carry out his duties as Earl Marshal, and chose for this post his brother Henry, who was officially appointed to the post in March 1816. On 14 October 1817 Molyneux-Howard resumed the use of Howard as his principal surname, becoming Henry Thomas Howard-Molyneux-Howard, and on the following day was granted a warrant of precedence to be styled as a younger son of a duke, and having thus gained the courtesy title "Lord", became known as Lord Henry Thomas Howard-Molyneux-Howard. He carried out his duties as Deputy Earl Marshal during the planning for the coronation of King George IV, but was unable by reason of illness to act at the actual event, at which his role was taken by his kinsman Lord Howard of Effingham. He was again returned to Parliament for Arundel from 1818 until 1820, in which latter year he was returned for Steyning, which seat he held until his death in 1824.
Marriage and children
On 12 September 1801 Howard-Molyneux-Howard married Elizabeth Long, daughter of Edward Long, a British colonial administrator, historian and author of The History of Jamaica, by whom he had one son and four daughters:
Howard-Molyneux-Howard died aged 57 in June 1824 after a short illness in his house in Lower Grosvenor Street, Mayfair. His eldest son Henry Howard replaced him in Parliament. The Teversall and Wellow estates, which had been entailed on his second son or eldest daughter, descended to his daughter Henrietta Anna Howard, later Countess of Carnarvon, who adopted the surname of Molyneux-Howard in consequence.