According to the United States Census Bureau, the city has a total area of 0.8 square miles (2.1 km²), all of it land.
There were 179 households out of which 29.1% had children under the age of 18 living with them, 54.7% were married couples living together, 10.1% had a female householder with no husband present, and 30.2% were non-families. 25.7% of all households were made up of individuals and 14.5% had someone living alone who was 65 years of age or older. The average household size was 2.41 and the average family size was 2.86.
In the city the population was spread out with 22.5% under the age of 18, 9.5% from 18 to 24, 23.4% from 25 to 44, 23.8% from 45 to 64, and 20.8% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 101.9 males. For every 100 females age 18 and over, there were 97.1 males.
The median income for a household in the city was $27,500, and the median income for a family was $35,125. Males had a median income of $25,333 versus $21,667 for females. The per capita income for the city was $13,044. About 10.6% of families and 16.6% of the population were below the poverty line, including 33.9% of those under age 18 and 4.5% of those age 65 or over.
Thomes mapped out the city in 1850, with First through Fifth Streets running north-south, and B through I Streets running east-west. First Street no longer exists; it was eroded away by the river. Tehama was one of the earliest California settlements north of Sacramento. The town initially thrived on the riverboat traffic.
When Tehama County was formed in 1856, Tehama was established as the County Seat. However, the seat was moved to Red Bluff, by county-wide election, the very next year, although various local stories have circulated about how Red Bluff "stole" its county seat status from Tehama. Tehama had a reputation of being somewhat more liberal and freewheeling than the rest of the county, being the last town to go "dry" before Prohibition, and a center for bootleggers and gamblers.
Tehama's population peaked in the 1890s, at about 2000 residents, including a sizeable Chinese quarter. The city was incorporated in 1906 when plans were being made for an electric railway through the Sacramento Valley; the railway was never built, but Tehama remains as the smallest incorporated city in California. A disastrous fire in 1908 combined with the decline of the riverboat traffic, caused the city to gradually lose prominence and population. The last businesses, a mini-mart and a bar, closed in the 1990s, and the buildings stand unsold, leaving Tehama as almost entirely a residential neighborhood.
Tehama is generally the first area in the county to flood during the rainy season, although flooding is now controlled by releases from Shasta Dam. This gives Tehama a distinctive appearance, since by city ordinance, houses must be raised above flood level, so most homes have high foundations and tall stairways to the front door.
Tehama is home to two parks: Habert Park on C Street, and Belbeck Park, which contains an old brick schoolhouse, now used for the Head Start program. The Tehama County Museum , housed in what was once the Masonic Hall, is located at the corner of C and Third Street.