It was painted in 1914 by the pioneering director of Glasgow School of Art (GSA), Francis Newbery, and presents a quite different man from the moustached fop familiar from black and white photographs. The colours are wartime sombre but the portrait reveals a vibrant character in a flowing black coat, clutching in his hand architectural plans for his masterpiece on Glasgow's Renfrew Street, the second half of which was completed five years earlier in 1909.
This Hunterian exhibition is the culmination of four years of research led by experts at Glasgow University and the largest study of Mackintosh's architectural works ever undertaken, and has led to a revaluation of this under-researched area of his output. It will delight Mackintosh fans, who are to be found in every airt and pairt of the globe. If you are new to his work - or perhaps only started to think about it after the devastating fire at GSA in May - you will be mesmerised by the dexterity of his draftsmanship and the purity of his approach to design.
There are original plans on show here, as well as large-scale pen and ink drawings of major buildings, such as the Art School, Scotland Street School and the former Herald and Daily Record buildings, which will blow your socks off. Strip away the myth and you have a genius artist who got a thrill from seeing the vision he had in his head of a building come to life.
And woe betide the bureaucrat who got in his way... There are some lovely real-life touches here, such as the Glasgow Minute Book, which couches the inherent tensions during the design and build of Mackintosh's Scotland Street School. As noted by the School Board at one meeting to discuss the work-in-progress, the tile work was said to be "of a somewhat unusual character", and that "he [Mackintosh] was not prepared to modify plans". Mackintosh's original scheme, complete with huge ground-floor window, is on display, alongside a revised one showing seven sash windows of conventional proportions.
By examining the 126 architectural works in which Mackintosh had a hand in designing - some as mundane as a job at 34 Carrick Street in Glasgow in 1893 which saw him draw up plans for complying with new regulations on indoor water closets - this project challenges the familiar view of Mackintosh as an isolated genius. Not only has it defined his role in the buildings produced by the firm John Honeyman & Keppie (before becoming Honeyman, Keppie & Mackintosh), it charts the evolution of his now ultra-familiar style, identifies previously unrecorded works and confirms previously uncertain attributions.
One such building is a small extension to 12 Clairmont Gardens in Glasgow's west end, which Mackintosh worked on during 1898/89. It has now been established that Oswald Fergus, dentist, angler and amateur poet, clearly saw the poetic genius of Mackintosh, and commissioned him to design a two-storey rear addition to an 1850s sandstone terraced house, consisting of a WC "Lavy" (sic), billiard room, laundry, washing house and coal house. It's not all glamour …
Mackintosh Architecture features more than 80 architectural drawings from the Hunterian and collections across the UK. Many have never been exhibited and are on display alongside specially commissioned film, exquisite models by Brian S Gallagher of BG Models in Biggar and rare archival material. The findings from the project are also available to the public via a new website, www.mackintosh-architecture.gla.ac.uk. The resource will not only be of great public interest, but will play a vital role in future conservation and restoration of Mackintosh's buildings.
If you want to catch a flavour of the genius behind the moustache and foppish bow, then immerse yourself in his world for a few hours.
Mackintosh Architecture, Hunterian Art Gallery, Glasgow (www.glasgow.ac.uk/hunterian, 0141 330 4221) until January 4. The exhibition will move to RIBA's London gallery from February 18 to May 23.