The McKinstry's Mills Historic District is a national historic district in Union Bridge, located in Carroll and Frederick County, Maryland. The district comprises the entirety of the settlement of McKinstry's Mills, a 26-acre (110,000 m2) hamlet consisting of six separate properties that were owned and developed in the 19th century by the McKinstry family, local millers. At the center is a 31⁄2-story grist mill constructed in 1844. Also included are the McKinstry homestead, built between 1825 and 1835; the residence of miller Samuel McKinstry, dated 1849; a store building of 1850; and two other small houses and a variety of outbuildings. There is also a 1908 Warren pony truss bridge.