Reginald (Reggie) Francis Joseph Fairlie was born at Kincaple, Fife on 7 March 1883, the second son of James Ogilvy Reginald Fairlie and his wife Jane Mary Buchanan. The Fairlies were very old Ayrshire gentry, originally the Fairlies of that ilk until the Fairlie estate was sold to the Boyle Earls of Glasgow: James Ogilvy Reginald Fairlie was the third son of James Ogilvy Fairlie of Coodham and was Privy Chamberlain to Pope Leo XII 1878-1903, and from 1904 to Pius X. He was closely associated with the Third Marquess of Bute, and when Bute bought the Falkland estate in 1887 Fairlie acquired the Myres Castle section of it, matriculating his arms as Fairlie of Myres in 1893. This strong Roman Catholic-old gentry background was to be the bedrock of his son's practice.

Reginald Fairlie was educated at the Oratory School in Birmingham because of its associations with Cardinal Newman (whom Leo had made a cardinal in 1879) and was as devout a Roman Catholic as his father. He was articled to Robert Stodart Lorimer in 1901 but his relationship with him was not a particularly happy one. Fairlie did not believe in classes or exams and Lorimer told him he 'would never make an architect because he was too lazy'. Fairlie in turn had reservations about Lorimer and some of his work, preferring that of John Kinross and the Roman Catholic architect Archibald Macpherson who was to become a close friend until his death in December 1927. Within Lorimer's office Fairlie got on best with James Smith Richardson, later principal inspector of ancient monuments who was similarly somewhat disaffected. 

After leaving Lorimer's office Fairlie spent some time in London and travelled in England, France and Italy before returning to Edinburgh to commence independent practice at 14 Randolph Place in 1908, immediately entering the Stirling Municipal Buildings competition. He was unplaced but it hardly mattered. Family connections and the quality of his draughtsmanship ensured the patronage of the 4th Marquess of Bute who financed the Roman Catholic churches at St Andrews and Troon, both late Scots Gothic: these were followed by the choir of the abbey church at Fort Augustus, Romanesque and built off the foundations laid by Peter Paul Pugin. On at least one occasion he made the journey from Myres to Fort Augustus on foot, sleeping outdoors en route; and during this early period of practice he made further walking tours of the churches of Devon and Dorset in 1911 and 1912. At that time he kept a falcon.

Fairlie's first significant country house commissions came in 1913-14, a tactful addition to a composite Victorian shooting lodge at Inverailort and the reconstruction of Kilmany in a Cape Dutch idiom after the client had a disagreement with Lorimer. But in the latter year he was called up and commissioned in the Royal Engineers, serving in France. His elder brother John Ogilvy Fairlie was killed in action on 25 September 1915 and his father died on 28 September 1916, a year after becoming Chamberlain to Benedict XV. The deaths of his father and brother resulted in Fairlie becoming heir to the Myres estate.

Fairlie resumed practice at Randolph Place after the war, joining forces with George Reid and James Shiells Forbes - who also had Roman Catholic connections - to compete successfully for the Northfield Housing Estate in Edinburgh, the partnership taking the title of Fairlie, Reid & Forbes. This was followed by another housing estate in Moffat and a small one at Auchtermuchty which was not built. But after completing the Edinburgh and Moffat schemes, which were much above average, Fairlie gradually withdrew from 1926, leaving Reid & Forbes to continue in partnership on their own (Post Office directories show the firms running concurrently in this year). Fairlie's practice then settled into its former routine of church and country house work, which was of such quality that it brought him election as ARSA in 1923: and that in turn resulted in him being admitted FRIBA on 15 May 1928, proposed by the RIBA Council. He took rather more interest in professional matters after that, joining the Council of the Edinburgh Architectural Association and the Board of Edinburgh College of Art.

In 1925 Fairlie and his partners moved to 7 Ainslie Place, which was to be Fairlie's house and office for the rest of his life, his partners Reid and Forbes eventually moving out in 1930. It has been said that Fairlie lived the life of a hermit but that overstates the reality of his semi-monastic existence. He never married, his household being managed by a quick-witted gentleman's gentleman, Robertson from Leith. Robertson cooked good simple food ('the bona temporalia'), scanned his newspapers and journals in which the items which might interest him were duly marked for his attention, and kept his books and clothes in order - Fairlie was always impeccably clad, even after sleeping outdoors. The private apartments were kept in a state of old-world Catholic gentility, his drawing room having a Sienese Madonna, a Flemish pietà, a tapestry and Chinese ceramics. The house served also as an Edinburgh base for his brother Gilbert Thomas Ogilvie Fairlie, who was priest at Birnam, and also lodged within no 7 were James Forbes Smith, formerly of John More Dick Peddie's practice, and from about 1931, the widow of the architect James Bow Dunn, although neither seems to have been part of Fairlie's inner circle of friends who tended to be entertained at this time. Although shy and somewhat diffident in public, and intolerant of any pretension, in private Fairlie was a brilliant raconteur with a profound knowledge of Scottish and Irish folklore and an amazing mastery of provincial dialects, his tales always being in good taste. Surprisingly he had no interest in music beyond that for the Roman Catholic liturgy, and much of it he positively disliked. Perhaps the closest of his friends was the painter Francis Campbell Boileau Cadell who shared his love of outdoor life and of Iona in particular. He had maintained a similar bachelor household with a gentleman's gentleman at no 6 since 1920. Later regular guests were the Roman Catholic writer George Scott-Moncrieff, Ian G Lindsay who became an apprentice with him in 1927 on his return from Cambridge, and the sculptor Hew Lorimer whose career he both helped and encouraged from the time of his father's death in 1929.

The office itself never had many staff. Charles William Gray, who joined Fairlie's office in 1923, recalled that Fairlie 'never hustled or bustled. Very few people ever saw him work … as a master he never taught. He just breathed inspiration. Nothing seemed to be difficult to him. Churches were born during the night. As a draughtsman he was unique. The way he was able to create a chef d'oeuvre with the minimum of lines was uncanny. His freehand sketching in heraldic work and animals was amazing.'

Although war service had left him in less robust health and the pressures of business compelled him to use trains more, Fairlie remained committed to walking rather than motoring and to sleeping outdoors. His profound interest in mediaeval, renaissance and late Stuart architecture brought membership and ultimately the chairmanship of the Ancient Monuments Board at which he and his friend Richardson sometimes found themselves at odds with Dr W Douglas Simpson - 'The embattled Prussian has defeated us again' (Simpson was half-German). Whenever possible he revisited the sites under discussion. Hew Lorimer described how when visiting a site Fairlie 'would descend on it in the earliest light of dawn after sleeping out on some neighbouring hillside, or else might have stopped short of his objective and watched nightfall enclosing it and the lights of the countryside going out.' Predictably these self-imposed hardships caused chest problems and left him with a persistent and severe cough which shortened his life.

In July 1931 Fairlie's mother died. He allowed Myres to pass to his younger brother James Ogilvy Fairlie who had married Constance Lascelles in 1922. He and his sister Margaret Elspeth Mary Fairlie then rented Inchrye Abbey, a romantic Georgian Gothic house near Lindores, for the shooting. There Lindsay and the other members of his circle were frequent guests.

In 1932 Fairlie was among the twelve architects nominated by Sir Giles Gilbert Scott, then President of the RIBA, for consideration by the selection committee for the new government buildings on Calton Hill for the National Library. This was a three-stage process which began with the new offices in Whitehall. This went to Vincent Harris in preference to Arthur Davis and Thomas Tait by the casting vote of the First Commissioner of the Office of Works, Major William Ormsby-Gore, because Harris had won an earlier competition for them. At the Scottish meeting on 11 December when Sir D Y Cameron represented the Royal Fine Art Commission for Scotland and John Begg the Royal Incorporation of Architects in Scotland, the Calton Hill project went to Tait rather than Davis in deference to national feeling on the issue. The meeting was then joined by Lord Clyde for the National Library's interest. Ormsby-Gore proposed Davis because Fairlie had no experience of such projects but was firmly outvoted. Building was delayed as a new sheriff court had to be built before the old one could be demolished. The design stage was fraught with difficulties, the Cockburn Association proposing that Bryce's Sheriff Court façade be retained - a proposition Fairlie dismissed as impossible - and conflicting advice on style which ultimately led to a consultation with Sir George Washington Browne representing the Royal Fine Art Commission. A model was made of the final American-Scandinavian classical modern scheme, but it was not built as he designed it, the capitals of the pilasters and the doorpiece being drastically simplified after his death.

In the 1920s and 1930s Fairlie became resigned to the qualities he sought becoming harder to achieve. Of the red brick round-arched style spawned by Westminster Cathedral for Catholic church buildings, he observed diffidently, if not wryly, that 'It seems to be what they want'; and as Scott-Moncrieff correctly observed, perhaps with the Marquess of Bute's 1920s enthusiasm for synthetic stone in mind, he was 'not always firm enough with clients but allowed them too easily to make him modify his better judgment'.

In 1933 Fairlie was elected full academician and in 1937 St Andrews conferred on him the degree of LLD for his restoration of the interior of St Salvator's Chapel. Thereafter his staff referred to him as The Doctor. Appointment to the Royal Fine Art Commission and the Forestry Commission followed and in 1938 he made an extended tour of Norway and Sweden with Lindsay, partly with the National Library in mind and partly to see the work of the preservation bodies there: by that date he had generously allowed the commission for the restoration of the abbey buildings at Iona, originally planned in 1931-32, to pass to Lindsay who had left to set up practice on his own in 1931 and had become Burnett Orphoot's partner in 1933. There was, perhaps, a certain regret on both sides that a partnership was not possible because of the obligation Fairlie felt towards Gray and the practice's dependence on Roman Catholic business, Lindsay being a High Church episcopalian. At a personal level the mid- to late 1930s were not a happy time for Fairlie. Cadell ran out of patrons, and had to move from Ainslie Place and eventually downsize his household in Warriston Crescent; he died in serious poverty in 1937. Ian Lindsay's wife, the Hon Maysie Loch, recalled that although Fairlie was the kindest and gentlest of men, he somehow did not fully appreciate that his closest friend and his faithful manservant could no longer afford decent meals: characteristically she took over. In 1938 Robertson died suddenly, his place being taken by his sister Mrs Cameron; and in 1939 the outbreak of the Second World War resulted in the abandonment of his largest country house, Baro in East Lothian, and the loss of his lease of Inchrye.

The building of the National Library was slow to resume after the war: only the steel frame had been erected when work stopped in 1939. During the Second World War he had no work other than the various boards on which he served, but he was well enough off to see it through. He was admitted FRSE in 1947 and it was a matter of some amusement to him that a man with so many letters after his name had never passed any examination. He was wont to observe that it was far easier to be given degrees rather than work like a slave for them. 

In 1948 Fairlie took into partnership his long-serving assistant Major Charles William Gray. The following year work commenced on the nave at Fort Augustus and in 1950 when work on the National Library and on Baro was about to be resumed he took into partnership Alexander Ritchie Conlon.

Fairlie died in St Raphael's Nursing Home, Edinburgh on 27 October 1952, and was buried in the Eastern Cemetery, St Andrews where a stone carved by Hew Lorimer marks the Fairlie graves. He left the then substantial estate of £64,855 15s 9d. His practice was continued under the title of Reginald Fairlie & Partners. His sister Margaret commissioned Patrick Nuttgens to compile a memorial volume to him, but she died on 14 June 1958 without seeing it in print.