William Kent, born in Bridlington in 1685, is now mainly remembered as a furniture designer, the first interior decorator, and a very competent, if uninspiring architect, scarcely ever as a painter.

In fact, he has had a consistently bad press as a painter. William Hogarth wrote that 'Neither England nor Italy ever produced a more contemptible dauber'. George Vertue thought that he was unqualified and untalented, promoted only for his ability to ingratiate himself with the nobility, especially Lord Burlington.

Richard Boyle, 3rd Earl of Burlington and 4th Earl of Cork

Horace Walpole, who based his account of Kent on George Vertue's unflattering biography, regarded his work as a painter as 'below mediocrity'. This has been the nearly unanimous verdict of posterity.

Yet, it should not be forgotten that Kent had trained as a painter in Rome. According to Vertue, who is probably a reasonably reliable source for this, after an initial apprenticeship as 'a coach painter & house painter' he migrated to London and travelled out to Italy in July 1709 in company with John Talman, son of the leading architect of the time, and Daniel Lock, who was, like Talman, a person of wide artistic tastes.

Lock later got to know Hogarth, a fellow member of the Free Society of Artists and founder governor of the Foundling Hospital, and is commemorated by a bust by Roubiliac in the chapel of Trinity College, Cambridge. Lock, whose bust is accompanied by an artist's palette and a lyre, was described as 'a man as deeply attached as anyone to architecture, sculpture, painting, music and all the fine arts'.

After spending five months in Florence getting to know the city, visiting churches and private collections, and meeting up with leading antiquarians and dealers, the group travelled to Rome in April 1710, where Kent lodged with the painter Giuseppe Pesci in the Strada Paolina, started studying painting in the studio of Giuseppe Chiari, a pupil of Carlo Maratta, and took weekly painting classes at the Accademia di S. Luca.

Although he spent a great deal of time ingratiating himself with young British aristocrats on the Grand Tour, helping them buy prints and works of art, and travelling round Italy visiting palaces and archaeological sites, one should not underestimate the seriousness with which Kent studied works of art and copied them, later doing his own work.

In June 1713, as was subsequently reported in The London Mercury, he came second in the annual painting competition organised by the Accademia di S. Luca for a drawing of the Miracle of S. Andrea Avellino. He was awarded a silver medal by the Pope. In his first two years in Rome, he copied Domenichino's S. Nilus Healing a Possessed Child, Guido Reni's ceiling fresco Aurora twice, once for Sir John Chester of Chicheley Hall and again for Burrell Massingberd, whom he had got to know when he was out in Rome, and a fresco of Night by Guercino.

In the summer and autumn of 1714, he travelled round northern Italy visiting all the best-known sites, including Florence, Bologna, Mantua and Venice with Thomas Coke from Holkham in north Norfolk, whom he had met in Chiari's studio.

Aurora and the Chariot of the Sun Driven by Apollo

After a programme of copying, he began to 'do things of my invention'. In April 1715, he wrote to Massingberd about how he was 'learning to paint in fresco for my master is apainting a ceiling' (there is a memorandum about its technique in his journal) and, on 20th July 1715, was painting 'Venus a conducting Helina to Paris out of Homer' and 'a Madonna Bambino and St Giuseppe' for the 'next shew of pictures' in Rome.

Kent obviously took his training seriously and devoted a great deal of time to it and was encouraged to do so by his supporters, who were paying for him to be in Rome in order to become a painter.

What this means is that by the time Kent returned to London in December 1719, he was vastly better trained and more deeply knowledgeable about Italian art than any of his British contemporaries, including James Thornhill who was still at work on the hugely ambitious murals of the Painted Hall at Greenwich. He knew and had studied the Italian collections for 10 years and he must have been able to talk about them with authority.

His first project was to undertake three ceiling paintings for the rooms of parade at the front of Burlington House. Since the names of these rooms have changed over time as well as the location of its main staircase, it is perhaps worth being as explicit as possible about this commission. The middle room, known as the Saloon or Great Room, has a Banquet of the Gods, a very ambitious, not totally satisfactory composition, but as far as possible in tune with the magnificent bravura work of Sebastiano Ricci, who had worked for Lord Burlington before returning to Venice in 1716.

The room immediately to the west, known until the rooms were turned into the Fine Rooms as the Secretary's Room (it was the Secretary's Office), now called the Slaughter Room, has an Assembly of the Gods. He also painted The Glorification of Inigo Jones, a smaller and less ambitious painting, now set into the ceiling of the nineteenth-century staircase.

People don't now pay much attention to these paintings – they are ceiling paintings – but they were undertaken with sufficient confidence and authority to have won him two further commissions, one for the Duke of Chandos's house at Cannons, north of London, and the other for Viscount Castlemaine at Wanstead to the east. Kent had arrived.

Proof of a widely shared belief that Kent was the best painter available at the time comes from the fact that George I – no less – commissioned him to undertake the painted decoration for the whole of Kensington Palace, agreeing a contract for the Cupola Room in February 1722, written in French for the king's benefit.

Although there were considerable reservations on the part of the Office of Works as to whether or not Kent was the best person to have been employed on this very ambitious and extremely prestigious (and lucrative) project, Kent went on to paint room after room, beginning with the Cupola Room, including a grand oval painting of Jupiter and Semele on the ceiling of the King's Drawing Room, a less successful oval painting of Mars and Minerva Presiding over the Arts and Sciences in the Privy Chamber, a huge and ambitious array of members of the court looking out over the balcony of the King's Staircase, and ending with the King's Gallery, a huge room of state facing out onto the garden which he completed not long before the King's death.

The quality of Kent's painting on these projects is variable, but, at its best, as in the central panel of the King's Drawing Room, it is powerful and effective and does not deserve the derision with which it has too often been regarded.

Ceiling Panel, King's Gallery, Kensington Palace

While he was organising and overseeing the painting of the state apartments at Kensington Palace, Kent was much involved in projects elsewhere. He was commissioned to paint a new altarpiece for St Clement Danes and another for St George's, Hanover Square, which survives, a perfectly respectable if somewhat saccharine painting which many parishioners may think is authentically Italian.

In 1725, he was employed to oversee the decoration of the state rooms on the piano nobile at Houghton, the country seat of Sir Robert Walpole, First Lord of the Treasury, where he would entertain huge gatherings of his political friends and allies.

King's Stairs, Kensington Palace, North Wall

The problem – or maybe it was the opportunity – for Kent was that he was at least as much in demand for the layout of the rooms and the orchestration of the whole mise-en-scène as he was for the ceiling and wall paintings. In particular, he must have been very closely involved in every aspect of the design of the interiors of Chiswick. The exterior was the responsibility of Lord Burlington himself and well within Burlington's sometimes sterile competence as a designer. But the interiors must have been much more in the hands of Kent.

What one has constantly to remember is that Kent, unlike Burlington, had spent 10 years in Italy: he knew the buildings, the art collections, the palaces and archaeological remains; he had visited and recorded his response to the Palazzo del Te; although he had only spent a day in Vicenza, he had very much admired the Teatro Olimpico.

He knew the work of Palladio first hand, not just as an architect of linear elevations, as recorded in books, prints and drawings, but he also knew and admired the work of more mannerist architects as well, having lived and walked the streets of Rome for so many years.

Architecture Admiring a Portrait of Inigo Jones

One has also to remember that Kent was permanently resident at Burlington House in Piccadilly, when he wasn't staying with his mistress, Elizabeth Butler, in Covent Garden. He was not exactly a servant. He was a member of the household, allowed to stay on in a room upstairs presumably on the grounds that he remained useful as an artistic advisor, a confidante in Burlington's artistic projects, like a resident librarian or chaplain, able to correspond with Burlington with jocular amity.

He was buried in Burlington's vault in Chiswick church and left items in his estate to members of the family.

Kent was despised by Vertue and Hogarth as a toady, someone who had made a name for himself by getting to know the right people in Rome, making a profession out of friendship with the great and the good, and landing lucrative commissions as a result of their influence at court. But toadies have their uses, and Kent had the virtue of an unrivalled knowledge of sixteenth- and seventeenth-century Italian art.

He should not be entirely overlooked and scorned for his skill as a decorative painter, and must be credited for his wider knowledge, advice and expertise in the planning and layout of grand interiors.

Charles Saumarez Smith, Professor of Architectural History at the Royal Academy and author of The Art Museum in Modern Times, published by Thames & Hudson