Architect and builder. Born into a family of noted architects and builders. He was the eldest son of architectural sculptor Alexander Mylne (d.1643) and grandson of John Mylne (d. 1657). Mylne was appointed Master Mason to the Crown in 1668, succeeding his Uncle John (1611-67). He rebuilt Holyrood Palace under the direction of Sir William Bruce (1630 - 1710). The reconstruction had been ordered by King Charles II (1630-85), but it is to Mylne more than any other that the Palace owes its current form.
Mylne made much money through the speculative building of houses in Edinburgh, often with no regard for the building codes. He purchased derelict property around the Royal Mile and erected courtyard buildings which were more spacious than the tall 'lands' built around narrow entries which had existed previously. The scheme proved popular and was much copied. Examples were Mylne's Court, which now comprises student residences for the University of Edinburgh, and Mylne's Square, later demolished to make way for North Bridge. The Bank of Scotland set up in Mylnes Square in 1695, before moving to Parliament Close and eventually the Mound. This was also where the ill-fated Darien Scheme was planned and the Articles of Union were signed and sealed in 1707.
His other works included an improved water supply for Edinburgh, Wood's Hospital at Largo, Leslie House for the Earl of Rothes, the new Cross of Perth, a single arched bridge over the River Clyde at Romellweill Crags (a few miles S of Roberton) and the Mylne Battery at Edinburgh Castle.
Mylne served on the Town Council of Edinburgh for a time and owned the estate of Balfarg (or Balfargie) in Fife. According to his memorial in Greyfriars churchyard, he had eight sons and six daughters. His eldest daughter married another architect James Smith (c.1645 - 1731), with whom Mylne worked on Drumlanrig Castle. With his death the office of Master Mason to the Crown came to an end, being replaced by a government department.