Hillsboro, the county seat of Hill County, is on Interstate Highway 35, U.S. highways 81 and 171, State highways 22 and 171, Farm roads 286 and 3267, and the Missouri, Kansas and Texas Railroad, fifty miles south of Fort Worth and fifty-five miles southwest of Dallas, in the central part of the county. Katy Lake and Lake Aquilla are both partially within the city limits. When Hill County was established in 1853, three locations were suggested for the county seat, but none was near the center of the county. A 220-acre plot was donated by Thomas M. Steiner a mile from the center of the county, and the community was called Hillsborough, in honor of Dr. George W. Hill, a surgeon from Tennessee; Hill was the first settler in Navarro County, which later became part of Hill County. In 1853 the first school in Hill County opened, and in 1854 Hillsborough received a post office. Before the last courthouse was built in 1890, several courthouses served the community. The first, built of elm poles, was replaced by a frame building in 1854. In 1872 a two-story brick courthouse was constructed; it burned and was replaced by a fourth courthouse in 1874.
In 1881 Hillsboro was incorporated. The railroad arrived in 1881. The Katy shops built in Hillsboro brought jobs to the community and remained there until 1930. In 1888 the St. Louis, Arkansas and Texas of Texas (the Cotton Belt) arrived in Hillsboro. Early schools included Hillsboro Military Academy, later called Hillsboro Academy, Patterson Institute, established in 1885, Prothro Kindergarten, and Carlyle Military Academy. These schools became less popular when public schools for White and Black children were built in 1886. In 1888 the post office changed the spelling of the town name to Hillsboro. By 1890 Hillsboro had a population of 2,000, six churches, a new courthouse, three banks, a cotton compress, several cotton gins, a flour mill, stores, an opera house, and two weekly newspapers, the Mirror and the Reflector. The railroad shipped livestock, grain, wool, and cotton. The daily mail stage to Whitney also took passengers for a one-dollar fare. By 1900 the population was 5,000, and in 1903 the Trinity and Brazos Valley Railway began serving the town. In 1909 a new city hall with an attached fire and police station was constructed, and some city streets were paved. In 1915 Hillsboro received its first charter.
By the 1920s the population had risen to 7,000. Education was improved with the establishment in 1923 of Hillsboro Junior College, one of the first municipal junior colleges in the state, as a part of the Hillsboro public school system. When it had financial problems in the late 1940s the county government refused to help, and the college closed in 1950. After the Katy shops were withdrawn from Hillsboro, families who worked for them moved to Bellmead, the location of the new shops. In 1936 Hillsboro had six manufacturers supplementing the area's primary occupation, agriculture. Manufacturing was mainly involved with cotton, but included printing and ice manufacturing. By 1948 thirteen new manufacturers, including new dairy processors and monument and mattress makers, had taken the emphasis off cotton-related industries. Hillsboro had 157 businesses; the area's primary economic resources were clay, building stone, cotton, corn, oats, cattle, and poultry. The population in 1950 was 8,352, but by 1960 it had dropped to 7,402, and the number of manufactures had dropped to thirteen. This drop was caused by the growing importance of synthetic fabrics, which caused a decline in the cotton market. The cotton gins, compresses, oil mills, and textile mills, all formerly important in the Hillsboro economy, closed. In 1964 the last train ran on the Katy tracks.
In 1962 Hillsboro became a home-rule city with a council-manager form of government. That year the junior college was reactivated as Hill Junior College, and in 1964 the Confederate Research Center, the Audie L. Murphy Gun Museum, and the Hill Junior College Press were established. By 1970 the population had rebounded to 9,900, and Hillsboro had 210 businesses and twenty-one industries, including printers and manufacturers of sheet metal, fertilizer, clothing, and concrete. In the 1970s the courthouse-square buildings were renovated, and throughout the 1980s they were featured on a Heritage League Tour, held at the same time as the city's art festival. The population of Hillsboro was 7,072 in 1990 and 8,232 in 2000. In the 1990s Hillsboro had a public library, a municipal airport, and several historic sites, including the M-K-T Railroad Station and the Hill County Courthouse and Jail. Annual events included the county fair in March, the Hill County Round-up and Rodeo, and the Pioneer Day Celebration and Parade. The Hillsboro Reporter was the local newspaper.