Evidence of early human settlement in the area now covered by Kaeng Khoi district is found in the archaeological site of Ban Dong Nam Bo by the
Pa Sak River, which revealed a late-prehistoric (iron age) settlement dated to 2,000–1,500 years before present, and the cave of Tham Phra Phothisat in the hills to the district's east, which features
Dvaravati-era Buddhist carvings tentatively dated to the 6th to 8th centuries CE. The town of Kaeng Khoi developed as an outlying population centre of
Saraburi during the
early Rattanakosin period (late 18th to early 19th centuries). It was, along with several outlying communities of Saraburi, mostly populated by forced
Lao settlers from
Vientiane, who had been brought as war captives following Siam's capture of the city in 1778 and again following the
Anouvong rebellion in 1828. Kaeng Khoi became an important trading outpost, as it was both a stop for smaller vessels traversing the Pa Sak (larger vessels could only reach as far as Pak Phriao, the current location of Saraburi town) and the beginning of the route through the jungles of
Dong Phaya Fai (later renamed Dong Phaya Yen) which lead across a mountain pass into the
Khorat Plateau. Kaeng Khoi was established as a separate administrative district—then known as a
khwaeng—in 1827. It would become an
amphoe (today's district) following administrative reforms in 1897. The office of the district was moved from the Pa Sak River bank to the near the Kaeng Khoi Railway Station in 1915, and then to its current location in 1962. In 1859, Vice-king
Pinklao had a palace built by the river near the town, and made annual seasonal visits until his death in 1866, after which it was demolished. It is now known as
Si Tha Palace after the subdistrict of its location. The Pa Sak passed through steep hills north and upriver from Kaeng Khoi, and it became known as a site for nature excursions, according to writings by Prince
Damrong Rajanubhab during the reign of King
Vajiravudh (Rama VI, 1910–1925). When the Northeastern Railway was built during the reign of King
Chulalongkorn, Kaeng Khoi became the site of a major rail facility, servicing locomotives used for the uphill climb into the plateau. Kaeng Khoi station, opened in 1897, would later also become a major junction, and the town further developed as a railway town. From the 1970s, a cement industry developed in Kaeng Khoi district, and licences were granted for extensive limestone quarries in the nearby hills. Industry has since grown to become a major component of Kaeng Khoi's economy, though tensions over development and the pollution it causes have also arisen among the population. ==Geography==