The origins of the Placentia Area Historical Society reach back to 1937 when its precursor, the Ancient Capital Historic Committee, was founded by individuals such as Magistrate Michael Sinnott. Close to thirty years later, in 1971, the historical society revitalised itself and the newly organised Placentia Area Historical Society was born. With its gaining momentum, in the following year, the Placentia Area Historical Society (PAHS) was incorporated. Its first president was Aleisha Phippard. During this inaugural period, the other members of the executive were Austin Collins, Diana Curran and Patricia Power. The directors of the society were Rev. Phillip Lewis, Rev. Thomas Moakler, Robert Greene and Bernice Malone. These individuals gathered together because of their mutual objectives regarding the people and events of the past in the Placentia area.

Since its inception, one of the central objectives of the PAHS has been to promote the study, knowledge and preservation of the history of the Placentia Area. Throughout the years, the PAHS has initiated and participated in many exhibits, events and activities. During the seventies, some of the PAHS members opened a temporary community museum. In subsequent years, the opportunity arose to develop a more permanent home for a museum in the former home of William O'Reilly. The house had been built for William O'Reilly and his family in 1902 when he was acting as Magistrate for Placentia. It is known that the Cahill family rented it around 1934/36 and that as a young boy Tom Cahill, who is now recognized in the museum for his creativity, lived there. The house eventually became the official residence for the Magistrates in Placentia. Although other Magistrates such as Michael Sinnott did not occupy the house, William Linegar, a former Magistrate lived in the house until his death in 1984. At this time, it was acquired by the Placentia Area Historical Society for one dollar from the Provincial Government.

After a significant amount of time and dedication by members of the PAHS, renovations and refurbishments were completed on the house. In 1989, the former home of William O'Reilly re-opened as the O'Reilly House Museum. The work of the PAHS members restored the building to its former, well-regarded place in Placentia's built heritage. Owing to the success of these refurbishments and renovations, the O'Reilly House was distinguished by the Newfoundland Historic Trust which awarded the PAHS with the Southcott Award in 1989. Three years later, in 1992, the Placentia Area Historical Society was registered as a charitable organisation. Since the early nineties, the PAHS has continued to advance, administer and assure the preservation, recognition and celebration of the deep history and heritage of the Placentia area.

Currently, following in the footsteps of past decades, the membership of the PAHS is again enjoying renewal and rejuvenation. As of 2010, the PAHS has updated the by-laws as well as establishing a mission statement that will guide future endeavours. One of the essential initiatives for the PAHS has been to establish a website that focusses on sharing information about the work and efforts of the PAHS, as well as providing a taste of the history and heritage that the PAHS seeks to celebrate, communicate and advance.