Onarga was originally laid out in the mid-19th century alongside the development of the railroad from Chicago. Because of its close proximity to Spring Creek, Onarga was one of the first settled areas of Iroquois County. The town grew rapidly, but the population began to level off by the mid-20th century and now maintains fairly minimal population growth. Onarga is often associated with its tree and shrub nurseries that have been in business for over 100 years, earning Onarga the nickname, "The Nursery Capital of the Midwest." Many fields surrounding the town bear rows of trees and bushes rather than corn and soybeans, which are otherwise ubiquitous throughout the region. The prosperous nursery businesses have led to a great amount of ethnic diversity in Onarga, as many Mexican and Mexican-American migrant workers have chosen to maintain roots in the community.