Ranulph Dacre was born on 23 April 1797, probably at Marwell Hall, Hampshire, England. He was the fifth son of Julia Hall and her husband, George Dacre. George Dacre was colonel of the Hampshire Light Fencibles and high sheriff of the county.
Ranulph Dacre entered the Royal Navy in 1810. On the Spartan he was a shipmate of Frederick Marryat, the future novelist, and saw service in the blockade of American ports in 1812–13. Dacre resigned his commission in 1816 to become captain of a schooner trading to the West Indies, Australia and the Pacific islands for Robert Brooks of London. His first visits to New Zealand were in 1824 as part-owner of the schooner Endeavour, which called at Whangaroa in February en route to Tahiti and again in July while transporting the London Missionary Society emissaries Daniel Tyerman and George Bennet back to Sydney.
From 1825 to 1831 Ranulph Dacre traded between Australia, New Zealand and London. He then settled in Sydney, and married Margaret Sea, daughter of James Sea of Sydney, on 12 September 1831 at Maitland. In 1834 he set up a mercantile and shipping agency in partnership with William Wilks.
Their intention was to exploit Pacific products such as whale oil, sandalwood, kauri timber, greenstone and flax. Dacre made frequent voyages to New Zealand for sawn timber and for spars for the British navy. He acquired land at Mangonui, Mercury Bay and Mahurangi for timber working, shipbuilding, trading and stock raising. He successfully negotiated with Maori for land and timber rights, and obtained skilled workers and kept them supplied, despite attacks from Maori and the destruction of buildings and a vessel.
Captain Dacre was a tall, spare figure with an air of authority. 'There was no man trading in the North more respected, and deservedly so, for his gentlemanly conduct, & upright dealing', according to Thomas Poynton, who first met him at Hokianga in 1839. In 1832, when supplies were needed, Dacre had travelled in an open boat from Mahurangi to Coromandel. On landing, one of his crew was killed by a chief. Dacre outfaced him and was offered compensation. He later recounted the story of another tense moment when his baby daughter Julia, dandled by Patuone, seized the great chief's topknot. This was a grave affront to Patuone's mana and retribution seemed imminent. However, after some deliberation Patuone decided to overlook the insult.
Dacre set up spar stations at Mercury Bay and at Mahurangi, where his difficulties included the appropriation of his trees by the captain of the naval vessel Buffalo. In 1836, at Mercury Bay, Dacre's associate, Gordon Browne, installed a water-powered timber mill, as well as a wharf and slipway. Dacre's New Zealand enterprises were not a financial success, but by 1840 he was one of the leading merchants in Sydney, owning a wharf, ships, a cattle station and other property, in addition to his New Zealand interests. He was appointed a magistrate, and was a director of the Union Bank of Australia and the Sydney Alliance Assurance Company.
In the depression of 1842–44 Dacre became insolvent, losing much of his Australian property. He set out to recover debts and property rights elsewhere in the Pacific. In New Zealand he and Browne laid claims to the lands they asserted they had bought before 1840. He soon began to prosper again, dividing his time between Sydney and Auckland, where in 1844 he established a business relationship with James Macky of the firm of L. D. Nathan and Company. In 1848 he was able to purchase the 3,334 acre Weiti block, south of Orewa, which his sons Henry and Life Septimus Dacre farmed as a cattle station. In 1854 he entered into partnership as a merchant and shipping agent with James Macky's brother Thomas and purchased more land, including about 4,000 acres at Omaha.
In 1859 the family moved from Sydney to Auckland. By this time T. Macky and Company was one of the largest firms in the city. The Dacres lived in Eden Crescent on the slopes of Official Bay. Ranulph Dacre was active in the Anglican church, a member of the Auckland diocesan synod in 1864, and a benefactor of St Paul's Church. Towards the end of his life he was once again wealthy and had provided amply for his children. By 1882 he and his family owned over 9,000 acres of land, valued at nearly £10,000. About 1878 Ranulph and Margaret Dacre had removed to England to obtain medical treatment for their daughter Harriett. They lived at Clapham, Surrey, where Ranulph Dacre died on 27 June 1884.