Thomas Lawrence: Regency Power and Brilliance, National Portrait Gallery

Thomas Lawrence was a child prodigy; from the age of 11 he supported his family by making pastel drawings of the fashionable elite who spent the season in Bath. The next step for an aspiring young artist was to learn how to paint in oils and Lawrence taught himself by doing self-portraits. He learned fast. The first painting in this exhibition of sumptuous portraits shows a diffident 19-year-old sitting sideways and glancing nervously towards us, as though fearful that his efforts will be laughed at.

Thirty-seven years on, we see him again (main picture). What a difference! He may have lost his hair, but he stares down at us with the calm assurance of a man at the height of his powers who, thanks largely to the patronage of the Prince Regent, has painted the chief movers and shakers of Regency England.

His first break came a year after that tentative early self-portrait in 1789, with a commission to paint Queen Charlotte (pictured below left), the wife of “mad” King George. It’s an incredible achievement. Traumatised by her husband’s mysterious illness and the Machiavellian manoeuvres of the Prince of Wales who, until the King’s recovery, had been jostling for power, the Queen granted him only one sitting, but did allow a lady-in-waiting to model her jewelry.

Unphased by her lack of co-operation, Lawrence produced a ravishingly sensitive portrait of the 45-year-old Queen, sitting beside a window overlooking the great park at Windsor Castle. Painted in filmy layers of white over pink and blue, her gown appears as flimsy as gossamer; this lends the slim figure a poignant sense of vulnerability that is confirmed by her expression of wistful introspection.

Perhaps for this reason, the Queen didn’t like the picture and refused to pay for it, so Lawrence showed it at the Royal Academy the following spring. It received a warm welcome from a public tired of the flamboyant, grand manner portraits perfected by the RA's director Sir Joshua Reynolds who, to his credit, was quick to recognise the talent of the young man already being tipped as his successor. (Lawrence was to become President of the Royal Academy 30 years later).

The ambitious newcomer had submitted 11 other pictures, including a portrait of the actress Elizabeth Farren standing in an Impressionistic landscape against a huge sky. The picture is animated by lively brushwork – such as the licks of white paint that give her satin cloak its vibrant sheen – and the young woman’s playful expression, which according to one critic perfectly caught her character: “arch, careless, spirited, elegant and engaging”.

Lawrence soon began experimenting with group portraits. Commissioned by the wealthy, "vain and capricious” Earl of Abercorn, his picture of the Earl’s mistress and illegitimate son is a dynamic swirl of motion. Reclining on a fur rug with a huge Newfoundland dog, mother and son are seen through a circular opening, as if the scene belonged to a utopian, chocolate-box realm of seductive fantasy that exists somewhere beyond the dull walls of normal propriety. Plump white flesh, rose-red lips, velvets, muslin, furs and succulent grapes add to the sensory overload of this intoxicating glimpse of ripeness and fecundity.

Lawrence was at his best, though, portraying men. After the Napoleonic Wars ended in 1815 he was commissioned by the Prince Regent to paint the sovereigns and military commanders who helped defeat Napoleon. He rose to the challenge, portraying Field Marshall Gebhardt von Blücher, leader of the Prussian forces, for instance, as a decisive man of action decked in military finery, commanding his troops against a dramatic sky full of billowing smoke, suggestive of the destructive fury of war.

The artist’s main strength and originality, though, lay in his ability to reveal the person behind the official mask. The Duke of Wellington is portrayed not as a military hero but a civilian; in his portrait the "Iron Duke" has become human, a man of intelligence and sensitivity.

Lawrence’s greatest portrait by far is of Pope Pius VII (pictured right), who was viewed at the time as a quasi-saint for his resistance to Napoleon and subsequent imprisonment. The 85-year-old pontiff is dwarfed by the tall throne and the papal robes enfolding his puny body. The sumptuous red of the velvets and the cream of his satin gown emphasise the sallowness of his complexion, yet his dignity is enhanced rather than diminished by the contrast. Lawrence’s great achievement was to portray him as an enduring symbol of passive resistance and as a frail old man. “The picture”, he wrote, “will be one of the best I have painted.” He was right; it stands up to comparison with Velásquez’s portrait, Pope Innocent X, painted nearly 200 years earlier in 1650, which is about as good as it gets.

• Thomas Lawrence: Regency Power and Brilliance at the National Portrait Gallery until 23 January, 2011