William Alexander Madocks (17 June 1773 – 15 September 1828) was a British politician and landowner who served as Member of Parliament (MP) for the borough of Boston in Lincolnshire from 1802 to 1820, and then for Chippenham in Wiltshire from 1820 to 1826. He is best known, however, for his activities as an agricultural improver in Gwynedd, especially around the towns of Porthmadog and Tremadog which he founded and which are named after him.

William Madocks was born in London on 17 June 1773 to middle-aged parents. He had two older brothers, John Edward, and Joseph, but his parents had suffered the death of two further infants after the birth of Joseph, who was ten by the time William was born. His father was John Madocks, a barrister at Lincoln's Inn, who would go on to become an eminent King's Counsel, and his mother was Frances. When he was christened at St Andrew's Church, Holborn, he was given the name William after his grandfather, and Alexander after Alexander the Great, rather than because it was a family name. Frances was the daughter of a London merchant called Joseph Whitchurch, who came from Loughborough in County Down, Ireland, although her mother was English and they lived at Twickenham.

The Madocks family had long associations with Wales, traceable back to the time of King Henry II, and William's father inherited property at Llangwyfan and near Wrexham. As he rose to prominence in the legal profession, the family moved to a substantial Jacobean house with its own private theatre in North Cray, Kent, as the Welsh properties were too far away. At the age of eleven, he went to Charterhouse boarding school, and spent five and a half years there, but left in December 1789, when it appears that the Founder's Day celebrations got out of hand, and William refused to submit to a flogging of the whole class. His father backed his stance, and he worked briefly in a country solicitor's office before going to university at Oxford. His father hoped that he might also pursue a career in the legal service.

While there was support for parliamentary reform, particularly among students, at the start of Madocks' university career, the events of the French Revolution resulted in Radicals being viewed with suspicion, and there was little support for reform. The Napoleonic Wars resulted in few of the gentry making grand tours of Europe, and travel to the remote parts of Britain, including the Lake District and Wales became popular. Madocks visited North Wales often, staying at the houses of the gentry. There was a tradition of house-parties and theatre, with Sir Watkin of Wynnstay holding a six-week season of plays each winter, at which Madocks and his brothers excelled. Joseph and William were noted for their duets, but the parties also offered lively discussion of land reclamation, landscaping and agricultural practices. Madocks' ideas on communities with a sense of purpose were shaped at this time by his brother's building of a house at Erith, where he also saw the benefits of reclaimed land, in this case former marshes by the River Thames.

In 1796 Madocks purchased the Dolmelynllyn estate, using inheritance from the death of his father. He paid £1550 for the farm and £950 for the timber and underwood. He created a ferme ornée (ornamental farm) there, where he entertained friends such as Thomas Love Peacock. He was attracted to the location due to its proximity to the waterfalls of Rhaeadr Ddu, Pistyll Cain and Rhaeadr Mawddach.

In 1798, Madocks bought the Tan-yr-Allt estate, on the western bank of Traeth Mawr, a large expanse of sand and tidal marsh which formed the estuary of the Afon Glaslyn. He set about extending his property by reclaiming Penmorfa Marsh from the estuary, and assisted by the surveyor and civil engineer James Creassy, who had experience of land drainage schemes in the Lincolnshire Fens around Boston, built a semicircular 2-mile (3.2 km) embankment, running parallel to the course of the river, to reclaim some 1,082 acres (438 ha) of land. The embankment was between 11 and 20 feet (3.4 and 6.1 m) high, and was made of sand, covered in turves. The project cost £3,000, and took 200 men with 150 barrows about six months to complete. He also supervised the construction of two catchwater drains and a large sluice, to drain the area behind the embankment. Madocks was growing wheat and rape on the reclaimed land in 1802, and planted barley and grass in 1803.

In 1800, the British government and the Irish government both passed Acts of Parliament which created the United Kingdom of Great Britain and Ireland. The Union with Ireland meant that there was a need for improved communication between the two countries, and Madocks was in favour of a route which crossed his estate, to reach Porthdinllaen, on the northern coast of the Llŷn Peninsula, which would provide the terminus for a ferry to Dublin. However, this route involved a crossing of the dangerous Traeth Mawr sands, near the mouth of the Afon Glaslyn, or a lengthy detour to the north to cross the river at the Aberglaslyn bridge. The alternative ferry route from Anglesey involved crossings of the River Conwy and the Menai Strait, bridges over both of which had not yet been built. Madocks therefore, emboldened by the success of his first embankment, revived a plan first proposed in 1625 by Sir Hugh Myddleton, and reconsidered in 1718 and 1770, for a more substantial stone-filled embankment across the mouth of the river Glaslyn. This would enclose some further 6,000 acres (24 km²) whilst providing safe passage across the estuary.

The Porthdinllaen Turnpike Trust Act was obtained in 1803, and in 1807 Madocks succeeded in steering the Porthdinllaen Harbour Bill through parliament. The improved harbour had been designed by the engineer Thomas Rogers, better known for building lighthouses. Madocks also began the building of a model town at Tremadog, which he planned himself. Madocks also promoted the building of turnpike roads, as part of his plan to open up the area and increase its prosperity. He was unable to devote all his time to his projects, as since 1802 he had been the Member of Parliament for Boston in Lincolnshire, and divided his time between Boston, London and Tan-yr-Allt. He needed someone to manage his projects, and although there were several capable candidates at Boston, he knew he needed someone with Welsh language skills and a Welsh temperament. John Williams, a gardener from Anglesey, had arrived in search of work in 1800, and having helped with the construction of the first embankment, went on to manage the gardens and nurseries of the Madocks estate. A partnership soon developed, and without Williams, most of Madocks' engineering schemes would probably have foundered.