The first people of the Tualatin Valley were the Atfalati or Tualaty tribe of the Kalapuya, who inhabited the region for up to 10,000 years before white settlers arrived. The valley consisted of open grassland maintained through annual burning by the Atfalati, with scattered groves of trees along the streams. The Kalapuya moved from place to place in good weather to fish and hunt and to gather nuts, seeds, roots, and berries. Important foods included camas and wapato, and the Atfalati traded for salmon from Chinookan tribes near Willamette Falls on the Willamette River. During the winter, they lived in longhouses in settled villages, some near what became Hillsboro and Beaverton. Their population was greatly reduced after contact in the late 18th century with Europeans, who carried smallpox, syphilis, and malaria. Of the original population of 1,000 to 2,000 Atfalati reported in 1780, only 65 remained in 1851. In 1855, the U.S. government sent the survivors to the Grande Ronde reservation further west.[9]

The European-American community was founded by David Hill, Isaiah Kelsey, and Richard Williams, who arrived in the Tualatin Valley in 1841, followed by six more pioneers in 1842.[10] The locality went by two other names—East Tualatin Plains and Columbia—before it was named "Hillsborough" in February 1850 in honor of Hill, when he sold part of his land claim to the county.[11] On February 5, 1850, commissioners chosen by the territorial legislature selected the community to be the seat of the county government.[11] Hill was to be paid $200 for his land after plots had been sold for the town site,[11] but he died before this occurred, and his widow Lucinda received the funds.[12] The town's name was later simplified to Hillsboro. A log cabin was built in 1853 to serve as the community's first school, which opened in October 1854.[13] Riverboats provided transportation to Hillsboro as early as 1867 when the side-wheel steamer Yamhill worked on the Tualatin River.[10]

Chief Kno-Tah is a large wooden sculpture depicting the face of a Native American. The sculpture is located in Shute Park, the city's oldest park.

In 1871, the Oregon and California Railroad line was extended to the area, but it ran just south of town because the city did not want to give the railroad land in exchange for the rail connection.[10] Hillsboro was incorporated as the Town of Hillsboro on October 19, 1876, by the Oregon Legislature.[14] The first mayor was A. Luelling, who took office on December 8, 1876, and served a one-year term.[15] Notable later mayors included Congressman Thomas H. Tongue (1882 and 1886) and state senator William D. Hare (1885).[15] On September 30, 1908, 5,000 people gathered as the Oregon Electric Railway opened a connection between the city and Portland with an interurban electric rail line, the first to reach the community.[16] In 1923, the city altered its charter and adopted a council-manager government with a six-person city council, a part-time mayor who determined major policies, and a city manager who ran day-to-day operations.[17]

A brick building was constructed in 1852 to house the county government, followed by a brick courthouse in 1873.[18] In 1891, the courthouse was remodeled and a clock tower was added,[19] and the building was expanded with an annex in 1912. A new courthouse replaced the brick structure in 1928. The last major remodel of the 1928 structure occurred in 1972, when the Justice Services Building was built and incorporated into the existing building.[18]

The city's first fire department was a hook and ladder company organized in 1880 by the board of trustees (now city council).[20] A drinking water and electricity distribution system added in 1892–93 gave the town three fire hydrants and minimal street lighting.[21] Hillsboro built its first sewer system in 1911, but sewage treatment was not added until 1936.[22] In 1913, the city built its own water system,[23] and the first