He was the son of Cornelius van Mildert, a gin distiller, and his wife Martha née Hill. Cornelius Van Mildert was the great-grandson of an Amsterdam merchant who migrated to London around 1670, Martha the daughter of William Hill of Vauxhall, Surrey, merchant and financier. William van Mildert was educated at St Saviour's Grammar School, Merchant Taylors' School and the Queen's College, Oxford. Loosely attached to the high church party, he was appointed Bishop of Llandaff from 1819 to 1826, a post which he held in commendam with the Deanery of St Paul's between 1820 and 1826, when he was translated to Durham. Prior to this, he had been rector of the church of St. Mary-le-Bow in London and Regius Professor of Divinity at Oxford, where he gave the Bampton Lectures for . Van Mildert is often described as a 'stormy petrel' on account of his outspoken expression of his views. As Bishop of Llandaff he broke with the practice of his predecessors and actually resided in the diocese. As the bishop's palace had fallen to ruin, he rented Coldbrook House near Abergavenny. During his time in Llandaff, he gained a reputation as "a conscientious diocesan". As part of the University of Durham's foundation, behind which he was the driving force, he gave Durham Castle to the university, where it became the home of University College. Auckland Castle therefore became the sole residence of the Bishop of Durham. In addition, he donated a large number of buildings on Palace Green, between the Castle and the Cathedral. These are currently in use by various departments of the university. Van Mildert was the last Bishop of Durham with significant temporal powers as a Palatinate Prince-Bishop. Those secular powers were removed and returned to the Crown after his death in 1836 by the Durham Act 1836. In truth, most of the secular power of the Bishop of Durham had already been removed by that time. The Great Reform Act 1832 saw the removal of most of the Prince Bishop's powers although he maintained a seat in the House of Lords and The Municipal Corporations Act 1835 gave the governing power of the town of Durham to an elected body.
