Mylne learned his trade from his father, assisting him with projects including the sundial at
Holyrood Palace. In 1633 Mylne was made a
burgess of the
royal burgh of
Edinburgh, and was admitted to the Edinburgh lodge of masons, both due to his father's position. He was first appointed to the town council in 1636 and, in the same year, was appointed master mason to the Crown, succeeding his father. His building projects were concentrated in Edinburgh, where, from 1637, he served as principal master mason to the city. For ten years he was involved in the construction of the
Tron Kirk on the
High Street, which opened in 1647. The Tron was built to house the congregation of St Giles', which had been raised to cathedral status, and was laid out in the new T-plan form with the pulpit in the centre, to suit reformed worship. The design was informed by contemporary Dutch architecture and, in particular, by the work of
Hendrick de Keyser whose
Architectura Moderna showcased his church designs in the Netherlands. Mylne worked on the building with master wright John Scott who was responsible for the timber work. The building was executed in a Dutch influenced style with both
gothic and
classical details. The church was not fully complete before Mylne's death and was subsequently remodelled in the 18th century. A new spire was added in the 19th century following a fire, but Mylne's work can be seen in the body of the kirk. The carved
tympanum was executed by Mylne's brother Alexander. From 1637 to 1649 he was also engaged on the design of
Cowane's Hospital in
Stirling, which was executed by Stirling mason James Rynd. Mylne also carved the statue of its founder for the facade. In 1642, Mylne surveyed the crumbling remains of
Jedburgh Abbey, for which services he was made a burgess of
Jedburgh. He built the choir, steeple, and north aisle of
Airth Old Church, commencing 15 July 1647. From 1643 to 1659, he served as master mason for the construction of
Heriot's Hospital (now a school), succeeding
William Aytoun. The building had been started in 1628 by
William Wallace, and would not be finally completed until 1700; Mylne rebuilt one or two of the towers in 1648. Also in 1648, Mylne was engaged to repair the
crown steeple of
St. Giles', Edinburgh. Projects in the 1650s included the building of fortifications in
Leith, and the addition of artillery emplacements to Edinburgh's town wall. He undertook the division of
Greyfriars Kirk, so that it could serve two congregations, and constructed a professor's house for
Edinburgh University, which was demolished in the 18th century. His design for the completion of the palace went unexecuted, with the work eventually being carried out by
Sir William Bruce in the 1670s. This house, demolished in 1950, resembled Heriot's Hospital and other Scottish 17th-century buildings, rather than looking forward to the new classical styles which would be introduced by Bruce. Mylne's architectural works are in the Scottish Renaissance tradition, which combined
gothic and
classical elements, together with
mannerist ornament, often derived from imported pattern books. Colvin describes Mylne as "the leading master of the last phase of Scottish mannerism". By the 1660s, Mylne's work was becoming old-fashioned, as the European-inspired Palladian began to be imported by William Bruce. ==Political and military service==