Thomas Brown was the son of Thomas Brown, the City Superintendent of Works in Edinburgh. He was articled to his father and probably spent some time in the office of William Burn as an improver, some of his early houses being very much in Burn's style. In 1837 he was appointed architect to the Prison Board of Scotland and by 1838 had an office at 3 North Charlotte Street independent of his father's.
In 1849 Thomas Brown II entered into partnership with James Maitland Wardrop. Wardrop was born in London on 16 March 1824, the son of James Wardrop MD, surgeon to George IV, and Margaret, widow of Captain Burn RN and daughter of George Dalrymple of North Berwick. He had been an apprentice to Brown prior to being taken into partnership. The practice of Brown & Wardrop was based at 19 St Andrew Square.
Wardrop gradually took over the design work relating to Brown's position as architect to the Prison Board of Scotland, including at the county buildings of Wigtown (1862), polychrome Franco-Italian Gothic and Alloa, Clackmannan (1863), Forfar, Angus (1869) and Stirling (design 1866, built 1874), all 15th/16th-century Franco-Scottish of the same school as David Bryce's Fettes. Indeed he may have spent some time in the office of David Bryce on whose work his domestic style and planning was closely based. In 1848 he had prepared a neo-Tudor scheme for completely remodelling Clifton Hall, Midlothian which was superseded by the executed scheme for complete rebuilding by Bryce in 1850.
Brown's daughter was born on 23 September 1853. On 28 September of the same year Wardrop married at Dundas Castle Anna Maria, 5th daughter of James Dundas, 24th and last of Dundas, a financially unsuccessful inventor. This widened the already extensive landed connections he had inherited from his mother. Much of his work consisted of modernising older houses, but in 1861 he secured the commission for the huge Scottish Baronial Lochinch, built for the 10th Earl of Stair as a setting for the collections of his countess, daughter of the Duc de Coigny. With a big tower as its dominant feature, well organised plan and indoor bowling alley and extensive formal gardens, it established him as a serious rival to Bryce, a position consolidated by the equally large and stylish Franco-Scottish Stitchill, Roxburgh (1866) and the remodelling of Callendar Park, Stirlingshire (1869-77) as a vast symmetrical Francois Ier chateau. Glenternie, Peeblesshire (1863), Ardwell, Wigtownshire (1869), Udny (1874) and Fairburn, Ross-shire (1877) were all of a similar school to Bryce's houses, but his substantial enlargement of the 16th-century Z-plan tower house of Nunraw, East Lothian (1868) in its own style with thick walls, small openings and convincing detail was by far the most accomplished essay in pure revivalism then achieved in Scotland, anticipating R S Lorimer's work much later; in similar vein was his rebuilding from ruins of Barnbougle, Dalmeny, Edinburgh (1881).
At Kinnordy, Angus (1879), Wardrop again broke new ground, the house being large and picturesquely composed in an early-17th-century Scots style but with a studied avoidance of towers, turrets, parapets and other baronial compositional features. At his largest and finest house, Beaufort, Inverness-shire, (1880) Wardrop again demonstrated, as at Stitchill, that he could handle the asymmetrical composition of a really enormous house better than Bryce by concentrating the design into large, simple masses. While Wardrop's classical work was usually subdued Italianate, he became a pioneer of neo-Georgian through his association with the London decorators Wright and Mansfield's Adam-inspired refit of Haddo, Aberdeenshire (1879) and the rebuilding of Barskimming, Ayrshire (1882) in a convincing late-18th-century idiom. Wardrop also rebuilt a large number of country parish churches in a distinctive early decorated style, notable Cumnock, Ayrshire (1864), Methlick, Aberdeenshire (1865), Stow, Midlothian (1862), and Ayton (1867) and Langton (1871) in Berwickshire; he was also a tactful restorer, as can be seen at Mid Calder, West Lothian (1863).
Brown appears to have retired or died in 1872 or 1873 (somewhat confusingly Thomas Brown II of Uphall died in that year but he does not appear to be related). Charles Reid, who had been chief draughtsman in the firm and was a brother of A & W Reid of Elgin, was then taken into partnership, although the practice does not seem to have adopted the title of Wardrop & Reid until 1874.