One of the area's most significant houses, Walnut Grove displays a half century of architectural evolution. The southern portion of the house, the oldest part, consists of two adjoining two-story log houses: the northern section is thought to date from c. 1780, the southern from c. 1800. Sometime around 1825, a twostory, hip-roofed brick addition, with an arched entrance and notable interior woodwork, was built to the north. To the west, this extension was joined at first-floor level to a formerly freestanding two-story log kitchen, giving the compound an L-shaped footprint. Still later, the log sections were clapboarded, and a two-story portico was built in front of the two earliest sections. The entire house is now painted white, providing a visual unity that belies the complicated construction history. A number of notable outbuildings remain, including a frame slave quarters and office, a brick smokehouse, and a stone and frame washhouse-infirmary.
The original log portions of the house were built by Michael Erskine, Jr., one of the area's first permanent settlers, whose future wife had been kidnapped by the Shawnee in 1799, held for four years, and ransomed just before her marriage. Andrew Beirne, who acquired the property in 1825, built the brick section, and members of his family still own Walnut Grove. A native of Ireland, Beirne was a merchant and entrepreneur who accumulated a vast fortune and was twice a U.S. congressman. The extensive farm that surrounds Walnut Grove's curtilage extends southward to the northern limits of Union, providing a clear line of demarcation between town and country.