William Duckitt was the son of the agriculturalist William Duckitt, inventor of several farming implements, and his wife Elizabeth, born Iles. William the younger became an assistant in the office of the treasurer-general in London. He married Mary Ann Whitbread in 1794 and they had three surviving sons. In July 1799 he was engaged by the secretary of state, Henry Dundas, to go to the Cape of Good Hope in order to introduce modern farming techniques there. He arrived in September 1800 with his family, at least nine assistants and workmen, seed of various cereals, farming equipment, and three head of Devon cattle. The party settled at Simonstown and made experimental plantings there while Duckitt inspected various properties in the interior. He finally selected the government farm Klapmuts as an experimental farm and received money to finance his agricultural experiments.
Duckitt's efforts to improve agriculture at the Cape included the introduction of a lighter and more efficient iron plough, invented by his father, to replace the traditional heavy wooden Cape plough which he found did not dig to a sufficient depth and did not break up the dry soil sufficiently. However, owing to its higher cost and resistance to change among the Dutch farmers it was not an immediate success. He advocated the use of manure on wheat fields and gardens, and tried to convince local farmers to breed cattle, horses and sheep of better quality. Particularly with regard to the treatment of merino sheep he provided valuable information that led to the improvement of local wool farming. He was a competent person who took his duties seriously and who had a strongly developed sense of honour in his dealings with others.
Duckitt wished to receive certain crown lands for his services and his claims were supported by the governor, G. Yonge. However, the Burgher Senate opposed the grant and nothing came of it. Yonge also awarded a contract to Duckitt and two Van Reenen brothers to provide meat to the troops at market related prices, despite objections by other officials. He also allowed Duckitt to use certain government farms, leading to a lack of a clear distinction between Duckitt's official agricultural duties and his private farming. After Yonge's recall to England in April 1801 because of corruption and mismanagement, acting governor Francis Dundas (a nephew of Henry Dundas, the British secretary of state) stopped further advances of money to Duckitt, expecting him to cover his own costs from the proceeds of his farming. The failure of Duckitt's crops in 1801 as a result of drought, and unauthorised expenses by him in April 1802 further strained his relations with Dundas. Towards the end of the British occupation in 1803 he was an embittered man and had achieved only limited success with both his farming methods and his advice to the local farming community. None the less he had become popular among the Dutch farmers and his ideas and methods gradually spread. His whole party therefore remained at the Cape, and signed an oath of submission to the Batavian Republic. In addition to farming, he bought and sold several farms, while he and his business partner John Watney sold agricultural implements and other merchandise in their store in Cape Town. In October 1812 Duckitt was appointed secretary to the newly appointed agricultural board and was in charge of its experimental station near Darling until the board was dissolved in 1815. Meanwhile he had developed into a successful private farmer. After 1815 he settled on Klavervlei, near Mamre, where he farmed with cereals, sheep and cattle, and bred race horses. His eldest son, also named William, continued to farm there after his death.