He was born in St. James's Street, London, on 10 June 1787, the posthumous son of a China merchant, who after some years' residence in the East had died about five months before his son's birth, leaving a widow with five infant daughters. Indulged and petted by his mother, Harlow was sent when quite young to Dr. Barrow's classical school in Soho Square, and subsequently to Mr. Roy's school in Burlington Street. He was for a short time at Westminster School, but having shown a predilection for painting, he was placed under Henry De Cort, the landscape-painter. He next worked under Samuel Drummond, A.R.A., the portrait-painter, but after about a year entered the studio of Sir Thomas Lawrence, P.R.A. This step is said to have been taken at the suggestion of Georgiana, duchess of Devonshire; but Harlow's natural affinity to Lawrence's style in painting would be quite sufficient to account for his choice. Harlow paid Lawrence handsomely for his admission and the right to copy, but according to the contract was not entitled to instruction.
Harlow now determined to devote himself to painting, and refused an offer of a writership in the East India trade made by his father's friends. He remained for about eighteen months in Lawrence's studio, copying his pictures, and occasionally drawing preliminary portions of Lawrence's own productions. A difference about Harlow's work for one of Lawrence's pictures led to a breach with Lawrence, and Harlow rendered reconciliation impossible by painting a caricature signboard for an inn at Epsom in Lawrence's style and with Lawrence's initials affixed to it.
Harlow henceforth pursued an original system of art education. He inveighed strongly against all academical rules and principles. Young, headstrong, and impatient of restraint, with a handsome person and amiable disposition, he was generally popular in society. He affected, however, an extravagance in dress far beyond his means, a superiority of knowledge, and a license of conversation which gave frequent offence even to those really interested in the development of his genius. His foibles led his friends to nickname him "Clarissa Harlowe." He worked, however, with industry and enthusiasm in his art. He possessed a power of rapid observation and a retentive memory which enabled him to perform astonishing feats, like that of painting a satisfactory portrait of a gentleman named Hare, lately dead, whom Harlow had only once met in the street. Though openly opposed to the Royal Academy, he was a candidate for the dignity of academician, but he only received the vote of Henry Fuseli.
He exhibited for the first time at the Academy in 1804, sending a portrait of Dr. Thornton. In later years he exhibited many other portraits. His practice in this line was extensive. His portraits are well conceived, and, though much in the manner and style of Lawrence, have a character of their own. His portraits of ladies were always graceful and pleasing. He was less successful, owing to his defective art-education, in historical painting, in which he aspired to excel. His first exhibited historical pictures were Queen Elizabeth striking the Earl of Essex, at the Royal Academy, 1807, and The Earl of Bolingbroke entering London, at the British Institution, 1808.
In 1815 he painted Hubert and Prince Arthur for Mr. Leader, a picture subsequently exchanged for portraits of that gentleman's daughters. In 1814 he painted a group of portraits of Charles Mathews, the actor, in various characters, which attracted general attention. It was engraved by W. Greatbach for Yate's Life of Mathews. Harlow received a commission from Mr. Welch, the musician, to paint a portrait of Mrs. Siddons as Queen Katharine in Shakespeare's Henry VIII. This was commenced from memory, but subsequently the actress, at Mr. Welch's request, gave the painter a sitting. While painting the portrait, Harlow resolved to expand the picture into the "Trial Scene" from the same play, introducing portraits of the various members of the Kemble family and others. Mr. Welch, though not consulted by Harlow concerning this change of plan, behaved generously. The picture was exhibited at the Royal Academy in 1817, and excited great public interest. It was neither well composed nor well executed, and owed much to the criticism and suggestions of Fuseli, whose portrait Harlow was painting at the time. Still, the portrait of Mrs. Siddons herself as the queen will remain one of the most striking figures in English art. The fine engraving of it in mezzotint hy George Clint has enhanced its reputation. The picture passed eventually into the possession of Mr. Morrison at Basildon Park, Berkshire. It was exhibited at Manchester in 1857.
Harlow's next picture, The Virtue of Faith, at the Royal Academy, lacked originality, and had less success. It was purchased by his friend Mr. Tomkisson, who divided it into pieces for the sake of the heads.
In 1818 Harlow, conscious of deficiencies in his executive powers, visited Italy for the purpose of studying the old masters. At Rome his personal gifts and accomplishments, and his remarkable powers of execution, made him the hero of the day. He was f??ted and flattered in every direction. Canova was especially attracted by him, and obtained for him an introduction to the pope. Harlow, however, worked very hard, and completed a copy of Raphael's Transfiguration in eighteen days. He was elected a member for merit of the Academy of St. Luke at Rome, a most unusual distinction for an English artist, and was invited to paint his own portrait for the Uffizi gallery of painters at Florence. He painted a picture of Wolsey receiving the Cardinal's Hat in Westminster Abbey, and presented it to the Academy at Rome.
His artistic progress in Italy was remarkable, but on his return to England on 13 Jan. 1819 he was seized with a glandular affection of the throat, which being neglected proved fatal on 4 Feb. He was in his thirty-second year. He was buried under the altar of St. James's, Piccadilly, and his funeral was attended by the eminent artists of the day. An exhibition of his principal works was held in Pall Mall. His collections, including many sketches, were sold by auction 21 June 1819.
Harlow is one of the most attractive figures in the history of English painting. His works only suggest what lie might have achieved. Many of his portraits have been engraved, and those of James Northcote, Fuseli, Thomas Stothard, William Beechey, John Flaxman, and others are highly esteemed. His own portrait, painted by himself for the gallery at Florence, was engraved for Ranalli's Imperiale e Reale Galleria di Firenze. A drawing from it by J. Jackson, R.A., was bequeathed to the trustees of the National Portrait Gallery in 1888 by the painter's nephew, G. Harlow White. Another drawing by himself was engraved by B. Holl for the Library of the Fine Arts. His own portrait is introduced in the background in the picture of The Trial of Queen Katharine. A portrait of the Prince of Wales (afterwards George IV) by Harlow was engraved in mezzotint by W. Ward.