The Dewey area was settled around 1863 by pioneer prospector, rancher and Indian-fighter King Woolsey (1832-1879), who founded the Agua Fria Ranch, then better known as "Woolsey Valley". Apache raids made life difficult for the early American settlers. The stage station and post office nearby was also named Agua Fria. By the early 1870s water diversions were being used to irrigate an extensive area of corn and other crops. The ruins of Woolsey's ranch house can still be seen between the old Black Canyon Highway and the Agua Fria River about one mile north of Humboldt.
The Agua Fria post office closed in 1895. When a new post office opened in 1898, the community was renamed Dewey, probably to honor Admiral Dewey's great victory that year at the Battle of Manila, but perhaps after a pioneer settler. Farming continued in a small portion of the area until 2006 when the last working farm was sold to developers. Today Dewey is a low-density residential area.
Humboldt was also settled in the early 1860s. The town was originally named Val Verde after the company that owned the local smelting operation. The town was renamed Humboldt in 1905 to honor Baron Alexander von Humboldt, who had predicted more than a century earlier that the Bradshaw Mountains would become a rich mining area By 1907 the population had reached 1,000. With two daily trains, business in the town boomed and the city decided to showcase their development by hosting a Labor Day celebration that year. The celebration featuring a parade on Main Street became an annual tradition, now organized by the Agua Fria Chamber of Commerce and held on the last Saturday in September.
The panic of 1907 caused the nearby Iron King Mine to close temporarily. After World War I, the smelter and mine closed again, and by 1930 the population of Humboldt had dwindled to 300. Humboldt had a second but smaller boom in 1934 when the mine reopened and produced $100 million in lead and zinc before its closure in 1968. The mine tailings are presently being reprocessed into iron-rich Ironite fertilizer. There have been questions raised about the lead and arsenic content of the fertilizer, but the company maintains its product is harmless. "The lead and arsenic are in forms that cannot escape into the environment. You can eat them and they'll pass right through you," said Rob Morgan, Ironite's executive vice president and chief operating officer. "They're not harmful." However, the EPA has recently posted a cautionary statement, warning that potentially harmful amounts of arsenic could be released from use of Ironite. Ironite is banned in Canada.
The railroad track which served the mine was removed in 1971. Today in Humboldt, a lone smelter smokestack remains overlooking the historic buildings on Main Street.
According to the United States Census Bureau, the CDP had a total area of 22.9 square miles (59.3 km²), all land.
There were 2,795 households out of which 16.8% had children under the age of 18 living with them, 63.8% were married couples living together, 5.3% had a female householder with no husband present, and 27.6% were non-families. 23.0% of all households were made up of individuals and 13.9% had someone living alone who was 65 years of age or older. The average household size was 2.25 and the average family size was 2.59.
In the CDP the population was spread out with 16.8% under the age of 18, 4.0% from 18 to 24, 16.3% from 25 to 44, 31.4% from 45 to 64, and 31.4% who were 65 years of age or older. The median age was 54 years. For every 100 females there were 96.1 males. For every 100 females age 18 and over, there were 93.9 males.
The median income for a household in the CDP was $36,839, and the median income for a family was $41,232. Males had a median income of $35,446 versus $22,484 for females. The per capita income for the CDP was $20,326. About 4.8% of families and 8.7% of the population were below the poverty line, including 14.9% of those under age 18 and 6.2% of those age 65 or over.