Architect and building contractor. The third is a line of architects of the same name, he was the eldest son of John Mylne (d. 1657), whom he succeeded as Master Mason to the Crown. Appointed to this post by King Charles I in 1636, he continued in it until his death.
Mylne was a prominent citizen of Edinburgh, becoming Burgess and Member of the Town Council. He was one of twenty-one commissioners who went to England in 1651 to discuss a Treaty of Union, but returned having made no progress some nine months later. He was a Commissioner from the City of Edinburgh to the Scottish Parliament in 1662 and 1668.
His building works around Edinburgh include the Tron Church (1637-63), the restoration of parts of St Giles, the completion of Heriot's Hospital, new buildings for the University of Edinburgh (then called the Tounis College). He also worked on Newbattle Abbey (1650), together with the fortification of Leith (1649-50). Mylne became a Freeman of Jedburgh in return for his report on the Abbey there. He is buried in Greyfriars churchyard.