[2] The first courthouse in the county was a log cabin near what is now Northwest 253rd Avenue, located on the land claim of Edward Constable where court sessions were held briefly.

[3] In 1850, David Hill sold 40 acres (160,000 m2) and a cabin from his land claim to the county for $200.

[3] This cabin was used to house the court until 1852, when a two-story building was finished to serve as the courthouse and center of county government in Hillsboro.

[3] Built of cedar, this building was on land donated by the Kelsey family and located on the same block as the current site of the county courthouse, on the northwest corner of Main and 1st streets in downtown Hillsboro.

[5] Construction began in the middle of 1852 and the county accepted the building on December 8, 1852, at a cost of $3,300.

[5] The brick courthouse was remodeled in the early 1890s when it was expanded and a clock tower added to the building.

[5] In 1912, the county contracted with Portland contractor William Foster to expand the building at a cost of $32,516.

This square has five large sequoia trees that were planted in 1886 with seeds from John Porter,[3][5] located on the south side of the building.