William Mylne (1734–1790) was a Scottish architect and engineer. He is best known as the builder of the North Bridge, which links the Old and New Towns of Edinburgh, Scotland. He was the younger brother of Robert Mylne, architect and designer of Blackfriars Bridge in London.
William was descended from the Mylne family of stonemasons and architects. He was the second son of Thomas Mylne, Deacon of the mason's guild, who built the original Edinburgh Royal Infirmary to designs by William Adam. Like his elder brother Robert, William attended the Royal High School. He then served time as an apprentice stonemason.
In autumn 1753 he left Edinburgh for Paris, where he studied at the École des Arts under the architect Jacques-François Blondel. As well as part-time study, he also undertook paid work, learning the art of marble carving, to supplement the minimal allowance from his father. The following year, at the suggestion of James Nevay, a fellow Scot in France, Mylne proposed to his father that he be allowed to go on to Rome to study architecture, with a view to eventually setting up practice in London. The result was that his elder brother Robert joined him in Paris, and they travelled to Rome together. Robert had recently completed his apprenticeship, and was preparing to take over the family business. He arrived in France by the end of November 1754, and the brothers set off a few days later. Travelling mostly by foot, due to lack of funds, they visited Avignon and Marseilles, where they took a boat to Civitavecchia, and arrived at Rome in early 1755.