On Monday 21 April 1760 Joshua Reynolds had a busy day. Through the morning and the afternoon he had a series of sitters. Each of these stayed for an hour in the painter’s premises on St Martin’s Lane and was no doubt ‘greatly entertained’ — as another of Reynolds’s clients recorded — by watching the progress of their portraits in a large looking-glass strategically placed behind the easel so the subject could view the artist at work. However, Mark Hallett suggests in this masterly and pioneering new study of Reynolds at work, the most interesting hour of the day would have begun at one o’clock.

That was the time at which the Revd Laurence Sterne arrived for his appointment. The resulting picture, now in the National Portrait Gallery, is a splendid example of the subject Hallett has set himself: Reynolds’s portraiture in action. This is not a biography — though it progresses from Reynolds’s birth on 9 July 1723, son of a clergyman in Devon — ‘about ½ an hour after 9 in the morning’ — to his funeral in 1792, the latter being a grand public occasion, with a lying in state in the Life-Room at the Royal Academy in Somerset House followed by a solemn procession to St Paul’s Cathedral. Neither is it a conventional study of Reynolds’s work. Rather this is an investigation of the strategies by which Reynolds achieved fame and status unprecedented for a British-born artist, including a knighthood, the first presidency of the RA, and the friendship of many of the most distinguished minds in London, among them Dr Johnson and the philosopher Edmund Burke.

The picture was not a commission from the writer but seems to have been painted as a joint venture on the part of Reynolds and Sterne, designed to enhance the reputation of each. In early 1760 Sterne was — to use a word that comes up frequently in describing Reynolds’s sitters — a celebrity. He had burst into prominence with the publication of the first part of his novel, Tristram Shandy, the year before.His portrait of Sterne is a case in point. It is in itself a brilliant performance, presenting Sterne as what he was – a clergyman – but also hinting at the anarchic imagination within. Sterne’s cleric wig is subtly askew, on his lips the slightly dangerous smile of a satyr in a bacchanal by Rubens.

Both artist and writer lived at the beginning of the modern age of public image and media fame. Reynolds’s paintings of well-known sitters were speedily turned into prints. Meanwhile, the works he showed in public exhibitions were carefully calculated to upstage his rivals. After 1768 these silent struggles took place at the infant Royal Academy — of which at the last minute and apparently against his better judgment Reynolds agreed to be president.

At the annual RA exhibitions in the late 1770s there were ding-dong contests between Reynolds and Thomas Gainsborough in the sphere of full-length portraits of beautiful women. It is hard to say who won. Early on, Horace Walpole offered the judgment, ‘Mr Reynolds seldom succeeds with women’, which turned out to be far from correct. On the contrary, Reynolds produced some wonderful female portraits, both sympathetic and sensual.

In several cases, but far from all, these were of courtesans — including Kitty Fisher, Nelly O’Brien, and the the actress Frances Abington depicted, in Hallett’s words, ‘leaning over the back of a chair and gazing out at the viewer with a thumb resting alluringly on her lower lip’. Equally sensational was ‘Mrs Robinson’ (1782), a picture of a poet and actress, much talked of after a love affair with the young Prince of Wales, whose look — clever, smouldering, defiant, calculating — still commands attention at a distance of nearly 250 years.

Gainsborough seldom gives you such an intimate sense of an individual, especially a female one, as this. On the other hand, he didn’t ever come a cropper in the way that Reynolds intermittently did. One of the qualities that made Reynolds so good was his ability to add ingredients to his portraits: painting Sterne at once as conventional clergyman and subversive satyr, for example. But he was also tempted to add extra value by blending his genres. Sometimes Reynolds stirred mythology into the visual mix, resulting in improbable pictures of English ladies impersonating classical nymphs such as ‘Lady Elizabeth Keppel Adorning a Term of Hymen’ (1761), ‘Lady Sarah Bunbury Sacrificing to the Graces’ (1762) or ‘Mrs Hale as “Euphrosyne” ’ (1762-4).

Even at the time, Reynolds’s more ambitious attempts at genre-bending were regarded as slightly preposterous. ‘The Archers (Colonel Acland and Lord Sydney)’ of 1769 — in which the subjects caper, dressed up as figures from a 17th-century hunting picture — is described by Hallett as teetering ‘on the edge of absurdity’. Of this, a contemporary correspondent in a London newspaper mildly noted that he had never known an instance ‘of partridges being killed by bow and arrows’.

Reynolds received a full critical fusillade, however, for his ‘George IV when Prince of Wales with a Black Servant’ (1787). No portraitist ever managed to make the future Prince Regent look less than slightly ridiculous, but he never seemed more preposterous than in this grandiose image of an overweight young man, wearing garter robes resembling an elaborate trifle (a journalist complained that at a public appearance young George looked like a ‘French footman’, an understandable response). Such was the virulence of the attacks that Reynolds gave up painting the Prince for good.

It was a rare defeat in a triumphant career, but one that in retrospect looks more mixed than it did at the time. Reynolds was a wonderful natural painter, but encumbered with intellectual and aesthetic ambition. He produced some of the finest portraits of the 18th century — pictures that bring you thrillingly close to Georgian men and women — but also quite a few of the silliest. The devices that grabbed attention at those long-ago Royal Academy exhibitions aren’t necessarily the ones that have lasted best (there may be a lesson for contemporary artists there). But through close reading both of pictures —whether successful or not — and contemporary texts, Hallett helps you peer into that studio looking-glass and see the painter at work.