Hachioji has a long history that the settlement of the lands stretches back into prehistory (the period before the invention of writing systems and history could be validated through written accounts).
Several historic sites designated by the government, such as the Kunugida ruins, the Funada ruins, and the Kitano ruins can be found in Hachioji.
A renowned military commander, Takeda Shingen's 6th daughter Matsuhime chose to move to Hachioji after the Takeda clan fell, and passed away at the Shinshoin located in Dai-machi. Takeda Shingen ruled the province of Kai (present-day Yamanashi Prefecture) during the era known as the Warring States period.
During this period, Hojo Ujiyasu's heir, Ujiteru established the Hachioji Castle as the Hojo clan expanded its forces throughout the area of Musashi from the Tama region.
The ruins of Hachioji Castle are now recognized as a significant tourist spot of great historical significance.
The present name of the city "Hachioji"was named after Hachioji shrine where Gozu-Tenno, the guardian god of Hachioji castle, his eight princes were enshrined at.
The castle and its surrounding area (the vicinity of Motohachioji-machi today) were claimed by Ieyasu Tokugawa after the Hojo clan was defeated by the Toyotomi clan.
When the Koshu Kaido road was being developed by Okubo Nagayasu, citizens living near the Hachioji castle lands were ordered to settle in the then newly-founded town, Hachioji-cho, which was established right alongside the Koshu Kaido road. A large inn-town subsequently formed along the Koshu Kaido road spanning several city blocks, which later on came to be known as the "Hachioji Juugo-shuku" (the 15 Hachioji Inns).
Tokugawa then conferred low-level ranks to ordinary farmers and formed a militia force called the "Hachioji Sennin Doshin" (Hachioji Thousand-man Militia). The militia was charged with law enforcement and ad-hoc military duties.
During this period, the area around the Hachioji Inns was a center of production for silkworms and the mulberries used to feed them. The area thrived as a relay point for raw silk thread until recently. Therefore the nearby road used to transport silk from Hachioji to the port of Yokohama is still called "Kinu no Michi" which is Silk Road in Japanese.
In 1889, the Kobu Tetsudo (present-day JR Chuo line) connecting Hachioji to Tokyo opened, and in 1917, second to the municipal government of Tokyo at the time, the municipal government of Hachioji was established which is set to celebrate its 100th anniversary in 2017.
In 1926, Yokoyama village was selected as the mausoleum site for the Emperor Taisho. The site was given the name Tama-no-Misasagi and a row of ginkgo trees were planted near the Koshu Kaido road.
Hachioji was designated by the government as a war-damaged city after World War II, and nearby villages were annexed to it. By 1964, the current demarcation lines for the city of Hachioji as we know it today was mostly complete, and the city's Ryonan Park became the venue for the 1964 Tokyo Olympics' cycling events.
Later on, the North Hachioji Industrial Park was established, luring precision machinery and electronics factories to set up shop, turning it into a thriving manufacturing hub.
In recent years, development of the Tama Center and the Minami-osawa area has been ongoing and the area has become well-known as a commuter town.