Greenfield is the second most populous city in the Salinas Valley and is the fifth most populous city in Monterey County. In 2006, Greenfield was the fourth fastest growing city in California growing 15.6%, from 13,270 in 2005, to 15,335 in 2006. As of the 2007 California Department of Finance estimate, there were 16,629 people, 2,643 households, and 2,360 families residing in the city. The population density was 9,781.76 people per square mile (3,779.32/km²). There were 2,726 housing units at an average density of 1,606.5/sq mi (619.1/km²). The racial makeup of the city was 39.65% White, 1.18% Black or African American, 1.19% Native American, 0.77% Asian, 0.15% Pacific Islander, 51.95% from other races, and 5.11% from two or more races. 87.86% of the population are Hispanic or Latino of any race.
There were 2,643 households out of which 65.3% had children under the age of 18 living with them, 70.5% were married couples living together, 13.0% had a female householder with no husband present, and 10.7% were non-families. 7.8% of all households were made up of individuals and 3.7% had someone living alone who was 65 years of age or older. The average household size was 4.75 and the average family size was 4.83. In the city the population was spread out with 38.3% under the age of 18, 13.4% from 18 to 24, 30.7% from 25 to 44, 12.5% from 45 to 64, and 5.0% who were 65 years of age or older. The median age was 24 years. For every 100 females there were 108.6 males. For every 100 females age 18 and over, there were 112.5 males.
The median income for a household in the city was $37,602, and the median income for a family was $35,520. Males had a median income of $25,759 versus $23,848 for females. The per capita income for the city was $9,226. About 17.1% of families and 21.6% of the population were below the poverty line, including 26.2% of those under age 18 and 11.8% of those age 65 or over.
In recent years, the town has seen a massive influx of immigrants from the Mexican state of Oaxaca. Many Oaxacans speak indigenous languages not related to English or Spanish and as a result, find communication with the large Spanish-speaking and smaller English-speaking population impossible. Probably the largest single block of such immigrants are speakers of the Copala Triqui language, who are fleeing from poverty and an armed conflict in their native region that has been going on for over 20 years. It is estimated that around 3,500 indigenous people live in Greenfield, or about 20% of the population.