The Magistrates of Placentia have been part of the governing of what is now Newfoundland and Labrador since 1729. In this year, Governor Henry Osborne sailed from Spithead, England to appoint Magistrates for six separate judicial districts: Bonavista, Ferryland, Harbour Grace, Placentia, St. John’s and Trinity. Until the actions of Governor Osborne, justice had been primarily administered by the Fishing Admirals, a role outside the Royal Navy Admiralty, but a title assigned to the first ship captain who entered a harbour. The changes instituted by Governor Osborne meant that justice would be shared by the Royal Navy, Fishing Admirals and the Magistrates.

As of 1729, the Magistrates of Placentia have presided over and been embroiled in the various judicial concerns that have taken place. In 1877, Thomas O’Reilly became Magistrate for Placentia. Alongside his role as Magistrate, he started the Star of the Sea Association in 1876 and acted as its President until his death in 1897. His son, William O’Reilly, then began his tenure as Magistrate.

In 1902, Magistrate William O’Reilly enlisted the duties of W.J. Ellis to design and build the O’Reilly House. That same year, Magistrate O’Reilly also oversaw the construction of the Placentia Courthouse. With recent changes, the Placentia Courthouse is no longer being used for judicial purposes. It closed as a court centre 31 January, 2009 and is currently a circuit court of Harbour Grace.

After William O’Reilly completed his tenure as Magistrate in 1923, he was succeeded by Michael Sinnott. Michael Sinnott is also well known as a founding member of the Ancient Capital Historical Society, an organisation that came into being in 1937. This Society is the precursor to the current Placentia Area Historical Society.

Following Magistrate Sinnott, in approximately 1939, William Linegar became Magistrate, a role he held throughout the governmental changes in the 1930s and 1940s and over the time when Newfoundland and Labrador joined the Canadian federation in 1949. Magistrate Linegar had an added legal challenge resulting from having a US Military base three miles from Placentia for just about all of his term in the local office. The fact that he was the first Placentia Magistrate to be a lawyer was undoubtedly helpful.

The O’Reilly House was taken over by the government as a home for the Magistrates of Placentia. Although Magistrate Sinnott did not reside in the O'Reilly House, it was a home to Magistrate William Linegar. The O’Reilly House served in this capacity until the 1970s and the death of William Linegar who had asked if he could continue his residence in the home after he retired as Magistrate in 1972.

At this time, the office of the Magistrate was taken over by Terrance Corbett until 1982. In 1979, by virtue of an amendment of the Provincial Court Act, the Magistrates became known as Provincial Court judges. The term judge applied to Terrance Corbett for part of his term in Placentia and to Gerald Barnable for all of his that started in 1982 and ended in 2004. Barnable is noted for writing a legal history of the “Ancient Capital” entitledUnder the Clock: A Legal history of the "Ancient Capital." He was the last such official appointed to Placentia where the court has now become a "circuit" for Carbonear.