At the date of Mylne's arrival in London designs for the construction of Blackfriars Bridge were being invited. Mylne, though a stranger in London, submitted one, which was approved in February 1760. His choice of elliptical arches in lieu of semicircular gave rise to some discussion, in which Dr. Johnson took part in three letters in the ‘Daily Gazetteer,’ 1, 8, and 15 Dec. 1759, in support of his friend John Gwynn [q. v.] It is to the credit of those concerned that the acquaintance thus formed between Johnson and Mylne developed later into a warm friendship, despite this difference of opinion. On 7 June 1760 the first pile of Mylne's bridge was driven. The first stone was laid on 31 Oct. (view of ceremony, from a contemporary print in Thornbury, Old and New London, i. 205), and it was opened on 19 Nov. 1769. During the years of construction Mylne was often abused and ridiculed, and the popular feeling was expressed by Charles Churchill in his poem of ‘The Ghost,’ 1763 (p. 174). A view of the approved design was engraved in 1760; an engraved plan and elevation by R. Baldwin, a view of a portion of the bridge by Piranesi in Rome, and another by E. Rooker in London, were all published in 1766. Mylne's method of centering has been much commended, and his design has been frequently engraved. Despite the fact that the bridge was constructed for something less than the estimate, Mylne had to resort to legal measures to obtain his remuneration. The bridge was removed in 1868.
Among Mylne's other engineering and architectural works may be mentioned: St. Cecilia's Hall in Edinburgh, on the model of the Opera House at Parma, since used as a school, 1762–5 (view in Cassell's Old and New Edinburgh, i. 252); a bridge at Welbeck for the Duke of Portland, 1764; the pavilion and wings of Northumberland House, Strand, 1765; Almack's (now Willis's) Rooms in King Street, St. James's, 1765–6; house for Dr. Hunter in Lichfield Street, 1766; Blaise Castle, Bristol, 1766 (views in Neale, Seats, vol. iv. 1821, and Brewer, Gloucestershire, p. 104); the Manor House, Wormleybury, Hertfordshire, 1767; the Jamaica Street Bridge, Glasgow, in conjunction with his brother William, noticed below, 1767–72; offices for the New River Company in Clerkenwell, 1770 (elevation in Maitland, London, Entick, 1775, vol. i. plate 128); Clumber Park, Nottinghamshire, 1770 (view in Thoroton, Nottinghamshire, iii. 405); City of London Lying-in Hospital, 1770–3 (Maitland, ib. vol. i. plate 127); Tusmore House, Oxfordshire (plan and elevations in Richardson, New Vit. Brit. vol. i. plates 3–5); Addington Lodge, near Croydon, since 1808 the residence of the Archbishop of Canterbury, 1772–9 (ib. vol. i. plates 32–3); the Bishop of Durham's portion of the bridge over the Tyne at Newcastle, removed in 1873 (Wooler being the architect of the corporation of Newcastle's portion), 1774; house for himself at the corner of Little Bridge Street, 1780 (cf. Thornbury, 'Old and New London, i. 207), afterwards the York Hotel, taken down in 1863, and the ground now occupied by Ludgate Hill railway station; works at Inverary Castle, 1780 and 1806 [see Morris, Robert, fl. 1754]; bridge over the Tyne at Hexham, Northumberland, 1784; hospital in Belfast, 1792; Mr. Coutts's house in Stratton Street, Piccadilly, 1797; the east front of the hall of the Stationers' Company, 1800; Kidbrook Park, Sussex, about 1804 (view in Neale, Seats, iv. 1821). He made considerable alterations to King's Weston, Gloucestershire, and Roseneath Castle, Dumbartonshire (1786), and repairs to Northumberland House in the Strand, Syon House, Middlesex, and Ardincaple House, Dumbartonshire.
Two of Mylne's great engineering designs were that for the Gloucester and Berkeley Canal, which has recently been completed to Sharpness Point, and that for the improvement to the fen level drainage, by means of the Eau Brink Cut above Lynn, which after much opposition was carried out by Rennie in 1817. Mylne drew up many reports on engineering projects, on which he was consulted. In 1772, after the destruction of the old bridge over the Tyne at Newcastle, he chose the site for a new one (many of his suggestions as to improvement in the approaches have been carried out in recent years); in 1775 he sounded the harbour and bridge at Great Yarmouth; in 1781 he surveyed the harbour of Wells-next-the-Sea in Norfolk; and in 1802 the Thames as far as Reading. In 1783 he reported on the disaster to Smeaton's bridge at Hexham; in 1784 on the Severn navigation; in 1789 on the state of the mills, waterworks, &c., of the city of Norwich; in 1790 on the Worcester canal; in 1791, 1793, 1794, and 1802 on the navigation of the Thames; in 1792 on the Eau Brink Cut; in 1799 and 1802 on the bed of the Thames in London, with reference to the reconstruction of London Bridge; in 1807 on the East London water works; and in 1808 on Woolwich dockyard. He was unsuccessful in his design for the new London Bridge in 1800.
Mylne was appointed surveyor of St. Paul's Cathedral in October 1766, and held the post
