William Leiper was born in Glasgow on 21 May 1839, the son of William Leiper who had a private school in George Street and traced his descent from the seventeenth-century Aberdeenshire master masons of that name. His mother was Jane Mellis, or Myles as it is given on Leiper's death certificate.
Educated by his father and at Glasgow High School, he served his apprenticeship with Boucher & Cousland from '1855-6' to about 1859 when he went to London, working for John Loughborough Pearson and William White for approximately one year each. There he gained an entrée to the circle of Edward William Godwin and William Burges, who was later to propose him as FRIBA. He was then for a time in Dublin supervising the building of Findlater Church for Andrew Heiton of Perth whose work for a time had similar qualities to Leiper's. Thereafter he found a place with Campbell Douglas & Stevenson in Glasgow. By very early 1864 he had formed a partnership with Robert Grieve Melvin who had inherited James Smith's practice on his death and completed Smith's Stirling's Library on Miller Street. About 1867 the partnership ended in a dispute, and Melvin gave up architecture for hotel keeping. Melvin's only significant work apart from Stirling's Library was the Glasgow Gas Company's palazzo at 42 Virginia Street, built in that year.
Leiper's reputation was immediately established by winning the competition for Dowanhill Church, Glasgow in 1864. Its spire drew inspiration from Pearson's design for St Peter's Vauxhall, G G Scott's scheme for St Mary's Stoke Newington (it was altered when eventually completed), thirteenth-century Rutland examples and those from Nesfield's 'Specimens' while its doorway gave Glasgow its first taste of Burges's Early French. Its interior was remarkable for its very wide single-span roof, probably inspired by Godwin's at Northampton Town Hall, and for its glass and stencilled decoration by Daniel Cottier. The adjoining hall was equally notable for the strong and simple Pearsonesque geometry of its roofs. The following year brought the richly sculptured Dumbarton Academy and Burgh hall, the design of which he had developed from Woodward's Oxford Museum and Godwin's Congleton Town Hall. In similar Early French vein were the towered extravaganza of Kirktonhill House, Dumbarton (1866, demolished), the smaller but more adroitly composed Cornhill, Lanarkshire and The Elms, Arbroath, Angus, the last towerless but with the high geometric roofs that were characteristic of him. In some of his less expensive early houses he adopted the low-pitched roofs and compositional methods of Alexander Thomson, a friend in his early years, notably at Bonnington (now Rhuarden), Helensburgh, Dumbartonshire, and Castlepark, Lanark, which has an unusual combination of Swiss and Anglo Japanese elements.
By 1869 Early French had been superseded by his own Frenchified version of Scottish baronial with lettered and sculptured towers and turrets at the compactly-composed Colearn, Perthshire, remarkable for its high quality aesthetic movement woodwork, Cottier stained glass with portraits of the daughters of the house, and Anglo-Japanese tiles by W B Simpson. Cairndhu, Helensburgh (1871) similarly had lavish Anglo-Japanese interiors of which a gold ceiling survives. Its exterior was a very early example of Francois Ier revival as was his Partick Burgh Hall, Glasgow, the following year, in which the French architect Alfred Chastel de Boinville may have been involved. Franco-Scottish remained the preferred style for his more ambitious houses, ranging from Dalmore, Helensburgh (1873), its larger version at Kinlochmoidart, Inverness-shire (1884), down to The Red Tower, Helensburgh and Glendaruel (1900-01) but a more conventional neo-Jacobean had replaced Anglo-Japanese for interior work by the 1890s. For the less affluent Leiper adopted a tile hung and half-timbered Norman Shaw manner that made its first appearance at his own Terpersie, Helensburgh in 1883, and sometimes achieved lavish proportions, notably at Piersland, Troon, Ayrshire (1898). These later houses became progressively more arts-and-crafts with perhaps an occasional glance at American essays in the idiom as at Morar (now Drumadoon), Helensburgh. Sometimes, as at Endrick Lodge, Stirling (c.1900) arts-and-crafts and Scots baronial themes were combined.
Leiper's church work similarly moved with the times. Camphill Church, Glasgow (1875-81) with its Caen spire was still Presbyterian in plan, albeit with masonry arcades, but at Hyndland, also in Glasgow (1886) a liturgical plan was achieved at the sacrifice of the intended spire. The saddleback tower of his last church, St Columba's, Kilmacolm, Renfrewshire (1902-05) still looked to Normandy, but to the flamboyant rather that Early French, with a tower derived from St Nicholas at Caen.
Leiper's commercial commissions were few but impressive. In 1889, as an architectural advertisement for their carpets, he built the polychrome red stone, brick and tile Venetian Gothic Templeton Factory on Glasgow Green and in 1893-94 the gargantuan Francois Ier Sun Insurance Building on Glasgow's West George Street on which William James Anderson and James Salmon Junior also worked and for which Birnie Rhind provided the sculpture, the French being sufficiently impressed to award him a Silver Medal at the Paris Exhibition of 1900.
From his earliest days Leiper was a skilful watercolourist and experimented with photography. In or about 1878 he took a career break to study painting. The reasons are not entirely clear and have been said to relate to disappointments in two limited competitions, which were presumably Woodlands UP Church 1874 where John Burnet, the assessor, made the final design drawing features from the competition entries, and Hillhead Established Church, where James Sellars was appointed to execute a design based on Leiper's suggestions, but his partner William Hunter McNab's statement that the City of Glasgow Bank Crash and consequent lack of business was the cause seems more probable. He studied in Paris at Julian's and then at R W Allan's with Arthur Melville. Although the worst years of the depression were still to come, the commission to design the interior of the Tsar's yacht Livadia in 1880 brought him home, his return to architectural practice being marked by his being admitted to FRIBA on 7 November 1881. His proposers were John Honeyman, Pearson and Burges. None of his paintings is in a public collection, but his numerous artist friendships, which included William McTaggart, brought him the commission to supervise the ambitious scheme of murals by The Glasgow Boys in the Banqueting Hall of Glasgow's City Chambers. Like Burnet's, Leiper's office is said to have had a studio atmosphere. The staff were often invited home, particularly at the time of the strawberry crop, and taken on a cycling tour.
Leiper was elected associate of the Royal Scottish Academy in 1891 and full member in 1896. From 1870 onward he exhibited works in watercolour and oil as well as architecture.
In or about 1899, Leiper formed a brief partnership with his former assistant William James Anderson for the commission for the interior work for SS Regele Carol I for the 'Roumanian Government'. It was run from Anderson's office at 95 Bath Street. As Anderson was in the middle of the inquest after the partial collapse of Napier House and the death of five workmen, Leiper must have been helping him with this particular job. Indeed it may have been that by this stage Anderson had been committed to Gartnavel Asylum.
Leiper never married. After a protracted illness which resulted from serious blood poisoning in 1903, Leiper was compelled to retire completely in 1909 and his practice was continued by his partner William Hunter McNab, an accomplished designer in the same idiom. Leiper died of a cardiac haemorrhage at Helensburgh on 27 May 1916. He left moveable estate of £6,535 10s 4d. A portrait of him by Colin Hunter hangs in Aberdeen Art Gallery.