Aristocrats, art collectors and former lovers gathered on the steps of St Paul’s Cathedral in 1830 as if paying respects to a dead war hero. In fact, they were mourning the portrait painter Thomas Lawrence. JMW Turner was among the crowd. His watercolour of the grieving public as the painter’s coffin is taken out of the hearse under the looming façade of St Paul’s is a masterpiece of Romantic history painting. For Turner, the lavish public send-off was proof that artists could achieve high status and popular acclaim. “It is something to feel that gifted talent can be acknowledged by the many who yesterday waded up to their knees in snow and muck to see the funeral pomp swelled up by carriages of the great,” he enthused.

Today Turner is far better known than Lawrence, the lion of society whose funeral so impressed him. Lawrence, it turned out, was too perfectly of his time for his own historical good. He was the star portraitist of the Regency era, patronised by the Prince Regent, later George IV, who sent him around Europe to portray the monarchs and prelates who were Britain’s allies against Napoleon. His masterpiece, however, is a full-length portrait of George himself, sitting in plump splendour at the Wallace Collection, looking raddled and dissipated in a great melancholy palette of blacks, greens and reds. This oneness with his moment made Lawrence look like a figure of the past to the Victorians. Thackeray in Vanity Fair and Dickens in Bleak House both dwell on his portrait of George IV as the defining image of its era. The Regency was by then mocked as a vanished age of moral decadence, and Lawrence appeared to be yesterday’s painter. Nor was the 20th century in any mood to do justice to the society painter of that foppish age.

Lawrence’s 1821-2 portrait of the young daughter of a Clapham wine merchant, offered by Bonhams in the Old Master Painting sale in December, proves the true feeling and poetry of this subtle painter. Jane Allnutt was four or five years old when she posed for Lawrence. He gave her an enduring image that is all the more poignant when you discover she died in her twenties. He sets the child and her dog against a dark blue night sky with silver clouds lit by the low moon in the trees. It is an oddly serious backdrop for a portrait of a little girl and her pet. Their faces are serious, too. While her cheeks are healthily pink and chubby and her hair is golden, Jane Allnutt’s eyes gaze at us with startling clarity. This is no cute kid, but a child of understanding beyond her years. Even her pet has a profound soul, its dark eyes full of the emotion early 19th-century Britons ascribed to dogs.

Sir Thomas Lawrence’s portrait of King George IV, 1822. Although Lawrence painted several portraits of the king, he regarded this as his finest. Copyright: Wallace Collection.

Sir Thomas Lawrence. Portrait of Queen Charlotte, 1789. Painted by Lawrence when he was only twenty years old. Copyright: National Portrait Gallery.

Thomas Gainsborough. The Painter’s Daughters chasing a Butterfly, c.1756. The painting depicts Mary (Molly) and Margaret (Peggy), the artist’s only children. Copyright: National Portrait Gallery

I draw your attention to the behaviour of the dog in this silvery night-time – for it is the clue to the painting’s true nature. This is a little Romantic poem in paint. Critics, from Thackeray to present day, who see Lawrence as the brittle artist of shallow Regency England, forget that he was also the contemporary of Wordsworth, Scott and Keats. This furry huggable canine cloud of sentiment ties this painting to the Romantics. Doggy enthusiasm reached new heights in the early 1800s. When a young man called Charles Gough disappeared on Helvellyn in the Lake District in 1805 and was found months later with his terrier barking beside him, Wordsworth and Scott both wrote poems about the loyalty of the pet. Wordworth’s is called ‘Fidelity’. Neither considered the likelihood that it survived on the bare mountain by eating its master, whose bones had been stripped bare.

If the Romantic movement was dewy-eyed about dogs, it saw childhood with acute insight and honesty. Portraits of children proliferated in 18th-century Britain after John Locke’s empirical philosophy popularised the idea of the ‘blank slate’ or, as he put it, white sheet of paper: born without pre-existing ideas, let alone original sin, the young are shaped by education and experience. Lawrence’s painting of a small child with bright eyes open to the world is in the tradition of Thomas Gainsborough’s The Painter’s Daughters Chasing a Butterfly. Gainsborough captured the curiosity and impulsiveness of his daughters in 1756, at the start of the Romantic age.

Lawrence takes that child-centred art to an almost disturbing pitch in this atmospheric portrait. It even has something in common with the ecstatic child portraits of the German Romantic painter Philipp Otto Runge. Lawrence’s insistence on the strength of mind and character of young Jane Allnutt makes you think of Wordsworth’s introspective early memories in The Prelude, or the peasant girl who eloquently refuses to accept her siblings’ deaths in his poem ‘We Are Seven’: “Two of them in the churchyard lie,/My sister and my brother;/And in the churchyard cottage, I/Dwell near them with my mother.”

Jane Allnutt was no peasant child, but the daughter of an affluent businessman. John Allnutt could afford to commission several other family portraits from the in-demand Lawrence, including another of Jane. Yet Lawrence knew he would appreciate this painting’s Romantic fervour. Allnutt was a discriminating collector with a taste for wild and melancholy landscapes by Turner and Constable. He owned Turner’s Devil’s Bridge, Mt St Gothard, which he intended to give to Jane – if he hadn’t outlived her. Its plunging vista of a perilous bridge and path teetering over an Alpine abyss is one of Turner’s purest distillations of the Romantic appetite for mountain horror.

If we don’t tend to see the bridge between Turner’s abyss and Lawrence’s faces, it is because we don’t look at portraits with open eyes. Lawrence was the last in a long line of great Georgian portrait masters – Reynolds and Gainsborough, Ramsay, Romney and Raeburn – who can all be dismissed by the hasty and insensitive yet all contain unexpected depths. Lawrence took the genre into full-fledged Romantic melancholia. His George IV is, in truth, uncannily insightful into its subject’s historical reputation as Britain’s worst monarch. More sympathetically, more nakedly tragic, he depicted Queen Charlotte, the Prince Regent’s mother, as a pale emotional wreck in front of an end-of-autumn Windsor landscape when her husband George III was temporarily insane. She hated it. Today, we can imbibe the nuances of tone with which Lawrence twists the Georgian portrait into a truly Romantic form: his deep and drunken colours, his eye for human weakness, and a brilliant merging of landscape and foreground that gets under your skin like a cold day.

The Allnutt family were more open to the Romantic daring of Thomas Lawrence than was the appalled Queen Charlotte. His portrait of Jane Allnutt must have seemed quite peculiar in its intensity. But her early death makes it ring true. Lawrence has preserved the face of someone who never quite grew up. She lives with his other ghosts, immaculate yet awkward, a face perceptive beyond its years. We are seven.

Jonathan Jones is the art critic of The Guardian

Sir Thomas Lawrence, Portrait of Mrs John Allnutt, c.1810-15. Eleanor Brandram was the second wife of John Allnutt, a wine merchant and art collector.

Sir Thomas Lawrence. Self-portrait, 1825. Probably begun before Lawrence left for Paris, this unfinished self-portrait may have been commanded by George IV. Copyright: Royal Academy.