According to the United States Census Bureau, the city has a total area of 10.2 km² (3.9 mi²), all land.
Mt. Diablo serves as a beautiful backdrop for Clayton.
In 1857, the town of Clayton was laid out and founded by Joel Henry Clayton (1812-1872) and his two younger brothers. Clayton was born in Bugsworth, now Buxworth, in the United Kingdom, and immigrated to the United States in 1837. After years in other states he settled down with his wife Margaret (1820-1908) at his town at the foot of Mt. Diablo, where he and his family prospered. Clayton was named after Joel Henry Clayton, although only by the flip of a coin. Joel Clayton and Charles Rhine cofounded the town, and each wanted to name it after them. If Charles had won it would have become Rhinesville, but Joel Clayton won.
The public schools of which the minors of Clayton attend include: Mt.Diablo Elementary School, Highlands Elementary School, Diablo View Middle School, Pine Hollow Middle School, and Clayton Valley High School.
As of the census of 2000, there were 10,762 people, 3,883 households, and 3,207 families residing in the city. The population density was 1,054.6/km² (2,729.3/mi²). There were 3,924 housing units at an average density of 384.5/km² (995.1/mi²). The racial makeup of the city was 87.95% White, 1.12% Black or African American, 0.19% Native American, 5.38% Asian, 0.10% Pacific Islander, 1.54% from other races, and 3.73% from two or more races. 6.33% of the population were Hispanic or Latino of any race.
There were 3,883 households out of which 39.2% had children under the age of 18 living with them, 74.6% were married couples living together, 5.5% had a female householder with no husband present, and 17.4% were non-families. 13.0% of all households were made up of individuals and 3.9% had someone living alone who was 65 years of age or older. The average household size was 2.76 and the average family size was 3.04.
In the city the population was spread out with 26.5% under the age of 18, 4.5% from 18 to 24, 28.5% from 25 to 44, 31.5% from 45 to 64, and 9.1% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 96.0 males. For every 100 females age 18 and over, there were 94.6 males.
The median income for a household in the city was $101,652, and the median income for a family was $107,448. Males had a median income of $77,535 versus $50,279 for females. The per capita income for the city was $42,048. About 1.2% of families and 2.6% of the population were below the poverty line, including 3.6% of those under age 18 and 1.9% of those age 65 or over.
A local superstition of Clayton is that of Sarah Norton. Sarah Norton, Henry Norton's daughter, was on the way to a funeral when, ironically, her buggy (horse-drawn carriage) derailed off the side of the road. She was buried in the Clayton cemetery and her ghost now "haunts" the cemetery and the road to it.
Actually, Sarah Norton was from Nortonville. She was a mid-wife. And she is buried at Rose Hill Cemetery in Nortonville (Black Diamond Mines). She is not buried in Clayton.