As an attached (i.e. official) architect within the Office of Works, he designed several major public buildings, including the main block and façade of the British Museum and altered or repaired others.

In May 1796, on the recommendation of George Dance the Younger, Smirke he began his study of architecture as a pupil of John Soane but left after only a few months in early 1797 due to a personality clash with his teacher.

In 1801, accompanied by his elder brother Richard, he attempted to embark on a Grand Tour, but was forced to return to England because war with France made it impossible to travel safely without fear of arrest.

The short-lived Peace of Amiens the following year allowed British travellers to visit France, and Smirke set off again in September 1802 in the company of the artist William Walker, returning early in 1805.

His itinerary and impressions are recorded in a series of letters and journals he wrote, many preserved in the archive of the RIBA, and in the many drawings he made of buildings and locations.

In France he visited places such as Paris, Lyon, Avignon, Nîmes, Arles and Marseille; he was particularly impressed by the various Roman monuments in the south of the country.

While in Italy, he passed through Genoa, Pisa, Florence and Siena but spent almost two months in Rome where he made the decision to visit Greece.

Turning south into the Peloponnese, he saw the famous sites of Messene, Megalopolis, Bassai and Olympia, before travelling on to Athens where he spent a month sketching the monuments.

He also managed to revisit Naples (seeing the ruins of Pompeii and Paestum) and Rome, as well as other cities, as he travelled up the Italian peninsula towards Venice, Padua, Vicenza and Verona.

Crossing into Austrian territory, he visited Innsbruck, Salzburg, Vienna, and Prague before moving on to Dresden and Berlin, returning to England via Heligoland in early January 1805.

His poor opinion of many of the more recent buildings he saw, even in Rome and Paris, combined with the overwhelming impact of ancient Greek structures, triggered a significant shift in his architectural tastes.

His knowledge of its ancient buildings, which were crucial to Ionic order widely used in the 1820s, especially the British Museum, was derived from publications such as Ionian Antiquities of 1769 by Richard Chandler, William Pars and Nicholas Revett.

During his Grand Tour, Smirke made drawings and watercolours of many buildings, including most of the surviving ancient structures in Athens and the Morea.

[6] He was elected an Associate of the Royal Academy on 7 November 1808, and a full Academician on 11 February 1811, his diploma work consisting of a drawing of a reconstruction of the Acropolis of Athens.

[9] He said: The practise of sacrificing everything to one front of a building is to be seen, not only in small houses where economy might in some degree apologize for the absurdity, but it is also apparent in large works of great expense ... And these drawings of a more recent work (here two drawings of Covent Garden theatre were displayed) point out the glaring impropriety of this defect in a manner if possible still more forcible and more subversive of true taste.

[13] This productivity inspired James Planché's 1846 chorus in his burlesque of Aristophanes' The Birds: Go to work, rival Smirke Make a dash, À la Nash Something try at, worthy Wyatt Plans out carry, great as Barry The rapid rise of Smirke was due to a combination of his great ability with significant political patronage.

[16] John Summerson described the design as demonstrating "how a plain mass of building could be endowed with a sense of gravity by comparatively simple means".

At the General Post Office in London in the mid-1820s he was still using the giant order of columns with a certain restraint, but by the time he came to design the main front of the British Museum, probably not planned until the 1830s, all such moderation was gone and he used it lavishly, wrapping an imposing colonnade around whole façade.

[17] Smirke, in the view of Charles Locke Eastlake, came third in importance amongst the Gothic Revival architects of his generation, after John Nash and James Wyatt, but criticised his work for its theatrical impracticability.

He said that his Eastnor Castle (1808–15), a massive, gloomy building with watch towers and a keep, "might have made a tolerable fort before the invention of gunpowder, but as a residence it was a picturesque mistake".

James Fergusson, writing in 1849, said "He was a first class builder architect ... no building of his ever showed a flaw or failing and ... he was often called upon to remedy the defects of his brother artist.

[24] Smirke was also a pioneer in the structure use of cast iron beams in non-industrial contexts, working in collaboration with leading engineers of the day.

This is initially seen in domestic buildings such as Eastnor Castle and Worthy House,[25] and then in larger projects, such as the 40-foot wide beams supporting the floors of the upper galleries at the British Museum;.

His pupils included Lewis Vulliamy, William Burn, Charles Robert Cockerell, Henry Jones Underwood, Henry Roberts, and his own brother Sydney who succeeded him as architect at the British Museum; although best known for the circular reading room at the British Museum, he added new galleries to his brother's original design in the same Greek Revival style.

The three bays on each side of the portico had arches on the ground floor and windows above these and a single carved relief above designed by John Flaxman.

The core design dates from 1823, and stipulated a building surrounding a large central courtyard (or quadrangle) with a grand south front.

Given the limited funds—which were granted by parliament on an annual basis—and the need to retain the Museum throughout the rebuilding programme, the work was divided into phases, and was subject to various changes before its completion over 25 years later.

Sydney Smirke also added polychromatic decoration in Greek Revival style to replace the plainer interiors designed by his brother, especially in the entrance hall and sculpture galleries.

The major surviving interiors are the entrance hall with the Great Stair – in the form of an Imperial staircase– rising to the west, and the "King's Library".

Nearly all Smirke's work was destroyed in the 1940–1941 London Blitz and has been rebuilt to a completely different design, the only major survival being the Paper Buildings of 1838,[38] in a simple classical style.