The former Mangonui Courthouse is a well-preserved judicial building of late nineteenth-century date. It was constructed during the early years of the first Liberal Government (1891-1912) in the maritime township of Mangonui. Mangonui was the seat of a resident magistrate, who was responsible for administering justice throughout much of the Far North of New Zealand. The earliest resident magistrate had been appointed in 1848, when the settlement had a reputation for lawless activity, partly due to its role as a port of call for visiting whaling ships. Erected in 1892, the timber structure replaced an earlier courthouse, which had been considered to be substandard. The new building was prominently located close to the post office and late nineteenth-century wharf, which provided the main point of arrival and departure from the town. The building was constructed as the Minister of Public Works, Richard Seddon (1845-1906), was in the process of re-organising the government's Public Works Department as a prelude to establishing a programme of nationwide works. The courthouse was part of the last generation of public structures built exclusively under the private contract system, with an increasing number of buildings from the mid-1890s being constructed by co-operative labour schemes that were directly administered by the state.

Although its plans were apparently drawn up by John Campbell - who was to become Government Architect in 1909 - the single-storey courthouse appears to have been modelled on an earlier, standard Public Works Department design. It was constructed by the local builder George Garton, who had erected other public buildings in the area. Of weatherboarded, kauri construction, the domestic style of the building is in contrast to the more forbidding appearance of courthouses in the larger towns and cities (see 'High Court Building, Auckland'), and bears strong similarities to nineteenth-century single-bay villas. A projecting bay contained the timber-lined courtroom, while a smaller room to the rear provided facilities for the magistrate. Another room at the front, capable of being accessed separately from the courtroom, may have initially accommodated customs work although it was later used by a clerk to the court. The courthouse was employed by the judiciary until 1948, after which it became a police station for 27 years. Taken over by the Mangonui Courthouse Preservation Society after a community campaign for its retention in the 1970s, the structure retains a large proportion of its original kauri furnishings. These include a dock and a judges bench, as well as a table made by local prisoners. It is now used as an art gallery (2002).

The former Mangonui Courthouse is nationally significant as an unusually intact example of a small court building, pre-dating widespread government construction from the later 1890s. It is among the last generation to reflect mid nineteenth-century attitudes to the creation of public buildings, notably through the private contract system. Its layout and interior demonstrate prevailing attitudes to justice and the social standing of the magistrate. The structure's prominent location in the town can be seen to indicate an ongoing state emphasis on law and order issues within rural communities, which continued when it was occupied by the police force. The structure also reflects Mangonui's role as an administrative centre and economic hub in the Far North. The courthouse is a valuable part of the historic urban landscape in Mangonui, and is of considerable value to the local community, as shown by the campaign to retain the building. It is associated with other late nineteenth- and early-twentieth century buildings along the foreshore, including the adjacent former post office and the Wharf Store.