The Old Town Hall was erected in 1762-64. It and the Cathedral are the most important buildings in the Old Town. A two-storey structure with a mansard roof, atop which is a small wooden clock tower. It is one of two 18th-century town halls still standing in Finland.

The building housed the town's administrative court, the council and the constabulary. It was also sometimes used for performances by itinerant

theatrical troupes and in times of war was converted into an infirmary. Its time in the limelight of history came in spring 1809, when the Diet of Porvoo was convened there. After 650 years as a part of the Kingdom of Sweden, Finland had just been annexed to the Russian Empire. To determine the future political status of his new possession, Czar Alexander 1 convened the Diet (the national legislative assembly comprising four Estates) in Porvoo. The Nobility and the Burghers met in two rooms on the upper floor of the Town Hall.

In structural terms, the building turned out to be something of a failure. Finnish builders had little experience of using stone as a structural material in those days, the foundations were inadequate and soon after its completion the building began leaning, which is especially noticeable in its interior. Nevertheless, the building continued to be used by the town authorities until the 1880s. In 1896 the newly-established Porvoo Museum Association saved it from demolition and opened a museum.

The building dates from 1762 and was once the home of a family of merchants called Holm. After the great fire that destroyed much of the town in 1760, several wealthy burghers, including Johan Holm Sr., had new houses built in brick. The Holm House originally had a mansard roof and its entrance was on the courtyard side. Business was conducted on the ground floor and the family lived upstairs. The attic contained small rooms for the servants. Several bakeries, including Swiss confectioners, operated in the building in the 19th century. To facilitate access to the bakery's retail outlet, a new door on the square side was opened in the 1860s. The Museum Association bought the building in 1919.

The property is on a slope and extends all the way to the next street, Vuorikatu. It includes two other small residences, in addition to several outbuildings. These are: a stable, a cowshed, a pigsty, a coach house, several barns and stores as well as privies. The outbuildings are not open to the public, but visitors are welcome to see the yard.