Alexander Marshall Mackenzie was born in Elgin on 1 January 1848, the son of Thomas Mackenzie, architect and his wife Helen Margaret McInnes. His middle name derived from his mother, who was a granddaughter of William Marshall, the Duke of Gordon's factor and a celebrated composer of reels and strathspeys. His father died in October 1854 when he was six. Educated at Elgin Academy, he was articled to James Matthews' Aberdeen office from 1863 to 1868, and remained there as assistant for a year. His elder brother Hugh being already settled in Edinburgh he then found a place in the office of David Bryce, living at 10 Forres Street. During that period he studied drawing and painting with Robert Innes who had painted a portrait of his father in 1851, and exhibited a selection of his topographical views at the RSA in 1870. This was, perhaps, at least partly in preparation for a study tour of Italy and France undertaken in that year, after which he commenced practice in Elgin at the early age of twenty-two.
By 1877, the year after Bryce's death, Marshall Mackenzie had amply demonstrated his capacity to gain clients, and Matthews was persuaded to re-admit him as a partner, but in respect of Aberdeen and Elgin-based business only, William Lawrie retaining his semi-independent position in Inverness where the practice continued under the name of Matthews & Lawrie. From 1883 onward Mackenzie undertook virtually all of the design work of the Aberdeen office, Matthews being preoccupied with civic duties as provost, principally on Rosemount Viaduct and the Union Terrace improvements. When William Lawrie died in 1887, his chief assistant John Hinton Gall took over the Inverness practice in his own name only. Matthews eventually retired completely in 1893 at the age of seventy-three, leaving Mackenzie as sole partner.
Marshall Mackenzie's classical work varied greatly in quality, mainly because of cost factors, working in granite being expensive. According to Herbert Hardy Wigglesworth, then his apprentice, a second visit to Italy in or about 1883 inspired the Northern Assurance Building and the Gray's School of Art and Aberdeen Art Gallery buildings, the details of the former suggesting that he had looked as much at modern Italian architecture as at high Renaissance examples. In the latter he adopted a two colour treatment by introducing elements of pink Corrennie granite, apparently in deference to the use of sandstone and brick dressings in Simpson's Triple Kirk opposite, an experiment that was to extend to the neo-Georgian villas he built in the 1890s. Much of his classical work from the mid-1880s onward was in a rather flat pilastraded idiom that lent itself to machine cutting: only at the Parish Council and School Board offices, and at the Manx Bank did he have the budget to adopt a more three-dimensional treatment.
Marshall Mackenzie's Gothic work was much more consistent in quality. From 1883 onwards beginning with Craigiebuckler, he paralleled Honeyman, Rowand Anderson and Blanc in the adaptation of mediaeval forms to a more liturgical form of Presbyterian worship. Both Craigiebuckler and Ruthrieston were English Gothic in detail, but thereafter he showed a marked preference for late Scots Gothic forms. This development stemmed from his restoration of Arbuthnott Church in 1889, but was also related to the Aberdeen Ecclesiological Society, originally initiated by the Rev James Cooper of St Nicholas East Church, William Kelly, later of Smith & Kelly, and his brother-in-law Charles Carmichael on Kelly's return from London and a continental study tour in 1885. Mackenzie was one of their founder members and his first new-build church in the Scots Gothic idiom was Powis, Aberdeen, 1895, its details drawn from Greyfriars Church, then under threat from the Marischal College extension scheme and which - against his own wishes - he was to be obliged to demolish.
Mackenzie was elected ARSA in 1893 although he had exhibited only once twenty-three years earlier, and admitted FRIBA on 30 November 1896 with the influential support of the London architects Alfred Waterhouse, Colonel Robert W Edis, and John McKean Brydon. These events were prompted by royal patronage, initially at the new church at Crathie in 1893 and again in 1895 when the Duke and Duchess of Fife (the Prince of Wales's daughter Princess Louise) commissioned the rebuilding of Mar Lodge. An honorary LLD followed in 1906, marking the final completion of the Marischal College extension scheme, formally opened by King Edward VII and Queen Alexandra to mark the Quater Centenary celebrations.
The completion of the Marischal College works brought the practice still greater national fame, but by then the practice had already opened a London office in 1903, a development directly related to Mackenzie's brother-in-law. Mackenzie had married Phoebe Ann Robertson Cooper, the only daughter of Alexander Cooper of the Elgin legal firm Cooper & Wink, and a granddaughter of General George Duncan Robertson, head of the Clan Robertson. Her brother George Alexander Cooper (1856-1940), later Sir George 1st Baronet, had become an American property magnate. He had also married an heiress, Mary Smith of Evanston, Illinois, the niece of 'Chicago' Smith, and became a major art collector, his dealer being Joseph Duveen. In 1901 the Coopers bought the lease of 26 Grosvenor Square, which made them neighbours to the Earl and Countess of Aberdeen at no 27: they had recently returned from Canada where Lord Aberdeen had been Governor General from 1893 to 1898. The London decorators Howard & Sons redecorated no 26 'under the aegis of Duveen Brothers' probably with some involvement by Duveen's architect Rene Sergent in Paris, the panelling for Duveen's tapestries being made by Anatole Beaumetz. While this work was in progress, Marshall Mackenzie's eldest son Alexander George Robertson Mackenzie - his middle names were those of his maternal grandfather- born 1879, was working with Sergent in Paris as an improver. Articled to his father in August 1894 at the age of fifteen, 'AGR' took classes at Gray's School of Art, Robert Gordon's Institute of Technology and the University of Aberdeen, and quickly developed extraordinary ability, becoming his father's chief assistant at the end of his apprenticeship in 1898. Nevertheless he felt he needed London experience, and early in 1900 he obtained a place in the office of one of his father's proposers, Colonel Edis, which enabled him to study at the Architectural Association and at the Central School of Arts and Crafts under Lethaby, Halsey Ricardo and Frampton. After his spell with Sergent in 1901 he obtained a place in the London office of Niven & Wigglesworth, Herbert Wigglesworth having been an apprentice of his father's, and passed the qualifying exam in June. He was admitted ARIBA on 17 September 1901, his proposers being his father, Wigglesworth, and his partner Niven. At that date he had travelled only in Normandy and in Holland, but soon thereafter he spent two months on a study tour in Italy before being recalled to his father's office in 1902 to assist with the Marischal College extension.
The London office was set up initially to enlarge and remodel Hursley Park in Hampshire, which the Coopers had bought in 1902, the work being carried out in association with Duveen, who obtained the boiseries and the Beauvais tapestries. AGR was put in charge of the London office although the division was by no means clear-cut, his father being in London for a few days every fortnight while the son undertook a certain amount of the design work of the Aberdeen office. Partly from the Coopers' influence and partly from sheer ability, the London practice was successful, at once securing the £300,000 commission for the Waldorf Astoria Hotel in Aldwych, followed by a still more prestigious one for Canada House, also part of the Strand-Kingsway improvements which put him in the same league as J J Burnet. The Canada House project was deferred but they won the competition for Australia House, also in Aldwych, a few years later. The lay assessors were headed by the sculptor Bertram Mackennal and were advised by J S Murdoch, formerly an assistant in Burnet's office and later Chief Architect in Canberra. This time the project went ahead. AGR was then elevated to FRIBA on 3 March 1913, his proposers being Leonard Stokes who had become a close friend and with whose son there was to be a future connection, and Niven and Wigglesworth.
By that date Gilbert Marshall Mackenzie was also in the London office. Born in 1890 or 1891 and educated at Charterhouse rather than in Aberdeen, Gilbert was articled to the Aberdeen office in 1909 but left in the same year for the University of Cambridge, probably to read modern languages in preparation for study at the Atelier Gromort in Paris in 1911. He returned to the London office in 1912 without taking the Diplome du Gouvernement, recalled to assist his brother with Australia House, and passed the qualifying exam in the same year without completing any apprenticeship and little more than a year's practical experience in total. He was admitted ARIBA in the same year, his proposers being his father, his brother, and another professional friend of his brother's, Herbert Austen Hall. Shortly thereafter he was taken into partnership.
The Mackenzies suffered severely in the First World War. The long-deferred Canada House project was cancelled, the Union Club and the Royal College of Physicians being eventually bought for the purpose and the commission given to Septimus Warwick. Gilbert was called up and commissioned in the First Battalion of the Seaforth Highlanders in which he reached the rank of Captain. While serving in France he drew and painted life in the trenches. Subsequently he was sent to Mesopotamia where he was killed in action near Kut on 21 April 1916. AGR enlisted as a private in the London Scottish, in the hope that he could transfer to the Seaforths and be with his brother, but was severely wounded and lost most of a leg. He was invalided out and assisted his father with the completion of Australia House, where work had continued throughout the war years. Mackenzie's second son, who had become a solicitor and was a partner in Cooper & Wink and was too old for active service similarly volunteered, but because of his eyesight he had to be content with the Service Corps from which he survived unscathed.
Alexander Marshall Mackenzie was elevated to the status of full academician in 1918, and the Aberdeen practice remained as prosperous as ever, but despite the continuing support of the Coopers, the London practice did not recover its pre-war success as Burnet's had done. Although still based in London, by the later 1920s AGR was spending much of his time on the work of the Aberdeen office, where Alexander Marshall was assisted by John Gibb Marr (born 1890), who had originally been articled to Clement George. Marr was taken into partnership on 1 January 1927. Niven and Wigglesworth's practice had also begun to run out of work following the completion of Hambro's Bank in London, and their partnership was dissolved in 1927, partly because Niven had developed other interests. Wigglesworth merged his half of the remaining practice with Mackenzie's. Further consolidation took place in Aberdeen in May 1931 when the Mackenzies merged the Aberdeen practice with that of the cinema and auction mart specialist, Clement George, born 1879 in Macduff, who had been in the office from 1897 to 1907, and had remained a family friend: his senior partner, George Sutherland had died in 1927. The practice now became A Marshall Mackenzie, Son & George.
These arrangements were to prove brief. Clement George died on 23 February 1932, followed on 4 May 1933 by Alexander Marshall Mackenzie who had been at the drawing board until within a week of his death, latterly working mostly from Culter House, a great early eighteenth-century house with a fine formal garden to which he had moved from the very stylish houses AGR had designed for him at Ladyhill and Loch Coull in 1911. The practice title then reverted to A Marshall Mackenzie & Son.