[2] Designed by Richard A. Waite,[3] the Ontario Legislative Building is an asymmetrical, five-storey structure built in the Richardsonian Romanesque style, with a load-bearing iron frame.

After the fire of 1909, however, the west side of the Legislative Building was repaired and expanded, with an added fourth floor that bears wall dormer windows in a long, gabled roof.

[3] At the far termini of the east–west axis, the wings each turn at right angles and extend north, enclosing a three-sided courtyard, in which sits the 1909 block, a free-standing, four storey structure that is rectangular in plan.

Inside, a central hall runs between the main entrance at the south and a grand staircase directly opposite, from the mid-landing of which is accessed the parliamentary library in the 1909 block.

From this core, wide corridors extend east and west, each bisected by a long and narrow atrium lined with ornate railings; the east wing is decorated more in the Victorian fashion in which it was built, with dark wood panelling, while the west wing corridor is more Edwardian Neoclassical in style, the walls lined with white marble, and reflecting the time in which it was built.

[5] The suite is a three-storey complex, with its own ceremonial stairway and elevator entrances, where members of the Canadian royal family and visiting dignitaries are greeted.

[6][7] The suite is also home to portraits of some past lieutenant governors (including a large rendition of Upper Canada's first lieutenant governor, John Graves Simcoe, painted by Edmund Wyly Grier and on loan from the Toronto Public Library[8]),[9] as well as of Elizabeth II, Queen of Canada, and Prince Philip, Duke of Edinburgh.

The Music Room is the largest space in the viceregal suite and is the site of New Years' levées, swearing-in ceremonies for cabinet ministers, and presentations of, and investitures for, provincial honours.

Only three years later, however, construction began on a dedicated parliament building in York (now Toronto), as it was felt by Lieutenant Governor John Graves Simcoe that the presence of a provincial capital directly across the border from the United States was too great a risk, especially as the relations between the US and Britain were then tense.

The relocation to York did not ensure the protection of the capital, however, and the Palace of Parliament was destroyed by fire on 27 April 1813, as a consequence of an attack on the city in the War of 1812.

In 1832, a new structure was built on Front Street, west of Simcoe Street, and served continuously as the third parliament building of Upper Canada until the province was united with Lower Canada in 1840, after which the joined assembly was relocated by the then Governor General, Charles Poulett Thomson, Baron Sydenham, to the general hospital building in Kingston.

With mounting displeasure over the transient nature of the Canadian parliament, and an inability on the part of politicians to agree as to where to locate the legislative building, Queen Victoria was asked to make a selection; over all the other cities in the Province of Canada, she chose Bytown (later Ottawa) in 1857.

As construction was underway, on 1 September men repairing galvanised roofing on the west wing accidentally sparked a fire that eventually destroyed the interior of that part of the edifice, including the legislative library.

[15] The infrastructure issues within the building include poor ventilation, the widespread presence of asbestos, not enough emergency exits to comply with fire codes, undrinkable tap water due to lead pipes, exposed wiring, and "mechanical systems that are close to, or already, failing.

"[16] In order to fix the "hazardous and severely deficient” conditions within the Main Legislative Building, a renovation requiring the full decamp of the staff and MPPs who currently work within the Legislative Building to an alternate location (similar to the renovation currently underway at Parliament Hill in Ottawa) is being planned, with a projected timeline of 10 years to complete once started and at a projected cost of more than a billion dollars.

[14][21] Due to these issues, the original goal of closing the building for the renovation by 2026 has been pushed back to at least 2028, and the former Minister formerly in charge of the project was quoted in March 2024 saying "the more you dig into it, the more comes up.”[16][14] According to a member of the Legislature's Procedure and House Affairs Committee involved in the planning of the renovation, a potential temporary relocation site has been identified and negotiations are underway, though the potential location has been kept secret.