Text from the plaque:
ONTARIO FIRST PARLIAMENT BUILDINGS 1798
In 1793 it was decided to move the capital of Upper Canada from Niagara to York (now Toronto). Two single-storey brick parliament buildings were constructed near this site. Opened in June 1798, the buildings were used for court proceedings and religious services in addition to parliamentary sessions. During their occupation of York, April 27 to May 2, 1813, American troops set fire to the parliament buildings. By 1820 they had been repaired and a connecting centre block added. Four years later, fire from an overheated chimney flue reduced them to ruins. The site was abandoned and in 1832 new parliament buildings were completed on Front Street, west of Simcoe Street.
From Wikipedia
First Ontario Parliament Buildings
The Front Street Parliament The first Ontario Parliament Buildings were built between 1829 and 1832 near Front, John, Simcoe and Wellington Streets in Toronto, Canada. Architects of the buildings were J.G. Chewett, Cumberland & Storm (firm), Samuel Curry, John Ewart, John Howard and Thomas Rogers. Alterations took place in 1849.The building later housed Upper Canada College and was demolished in 1903.
The buildings were originally the third constructed for the parliament of the province of Upper Canada, which sat in them from 1832–1841. In 1841, Upper Canada was absorbed into the Province of Canada, and its capital rotated among several locations. The buildings served as the seat of government for the new province at various times between 1849–1859.
When the province of Canada ceased to exist with the creation of Ontario at Confederation on July 1, 1867, Toronto was named the provincial capital, and these Front Street buildings were the first used by the Ontario legislature. Therefore, they are the province of Ontario's first parliament buildings, but not the first parliament buildings in present-day Ontario.
The Front Street location served the Ontario legislature from Confederation until 1893, when grand new parliament buildings opened at Queen's Park.
The site of Ontario's first parliament buildings is now occupied by Simcoe Place and the Canadian Broadcasting Centre