Thomas Mackenzie was born in 1814, son of Alexander Mackenzie, architect, St Martins, Perthshire, and his wife Janet Davidson. The precise date of his birth is not known, but he was christened on 9 October. He was articled to his eldest brother William Macdonald Mackenzie, and was in the office of his brother David in Dundee in 1835 when he submitted a design for the Watt Institution there, which was not accepted, George Angus's being preferred. In the same year Thomas moved to Aberdeen and would appear to have been briefly in the office of John Smith before moving to Archibald Simpson's, still in the same year. He remained there until 1839, publishing lithographs of Simpson's Marischal College, Market Street and New Market and of St Nicholas spire. In 1839 he moved to Elgin as principal assistant to William Robertson, who died in June 1841, his practice being inherited by his nephews Alexander and William Reid. Mackenzie then commenced practice in Elgin on his own account. He was successful at once, securing the commission for Elgin Museum, and went on to become an extremely accomplished classical and Italianate architect in a style developed from Simpson's late work.
In 1844 he formed a partnership with James Matthews, initially with Mackenzie doing most of the designing in Elgin, and Matthews attending to the management of the Aberdeen office. James Matthews was born in December 1819, son of Peter Matthews, a teller in the Commercial Bank in Aberdeen. His mother was Margaret Ross, daughter of William Ross, the architect-builder who had built Union Bridge. Educated at Robert Gordon's, he was articled to Archibald Simpson in 1834, and worked under Mackenzie's supervision. In 1839 he went to George Gilbert Scott's in London. On his return early in 1844, Simpson offered him the post of chief assistant with the promise of partnership in two years. He declined as he thought Simpson would be 'too greedy' (the Mackenzies, however, found Matthews 'a bit of a Jew'), and instead formed his partnership with Thomas Mackenzie. In the year of the formation of the partnership, Mackenzie & Matthews won the competition for the Free Church College (New College) in Edinburgh, in a competition assessed by Sir Charles Barry. The perspective, formerly at Bourtie, is now in the possession of Professor Alistair Rowan. The competition was set aside, however, and the commission given to William Henry Playfair. Initially the Elgin practice was much more prosperous than the Aberdeen one and in 1848 Matthews applied unsuccessfully for the post of head of the Edinburgh office of the Office of Works. The practice managed to secure the job of producing a prototype design for poorhouses.
On 22 July 1845, Mackenzie married Helen Margaret McInnes of Dandaleith, Rothes, at Rothes. The house and office was at Ladyhill, Elgin, to which he made Romanesque and baronial additions. In the late 1840s the London architect Robert William Billings became a particular friend when working on the Baronial and Ecclesiastical Antiquities of Scotland, their common interest in the subject resulting in Mackenzie being a particularly literate exponent of the Scots Baronial idiom.
Mackenzie died of brain fever - apparently brought on by an accident - on 15 October 1854, Matthews continuing the practice thereafter under his name alone. Just before Mackenzie's death an Inverness office had been established with William Lawrie in charge as resident assistant. Although not made a partner until 1864, Lawrie was given what seems to have been a free hand in the design work and for some years the Inverness office was the more prosperous. Matthews continued the Aberdeen office alone, and it was not until 1877 that Mackenzie's son, Alexander Marshall Mackenzie, was taken into partnership, having established a successful practice of his own in his native Elgin. Thereafter Matthews ran the practice as two separate partnerships - Matthews & Mackenzie in Aberdeen and Elgin, and Matthews & Lawrie in Inverness. When Lawrie died in 1887, the Inverness practice was inherited by John Hinton Gall (born 1848), who had been his chief assistant since 1872 and who continued the business under his own name, Matthews withdrawing completely from that branch of the firm.
Matthews entered the Town Council in 1863, and retired as a councillor in 1871. In November 1883 he was recalled as Lord Provost and held office until November 1886. He was mainly responsible for implementing the City Improvement Act of 1883. He was a director of the North of Scotland Bank, and its Chairman from time to time. He retired from the practice in 1893, and died at Springhill, which he had greatly altered for himself, on 28 June 1898.