Fullerton is a city located in northern Orange County, California, United States. As of the 2020 census, the city had a total population of 143,617. Fullerton was founded in 1887. It secured the land on behalf of the Atchison, Topeka and Santa Fe Railway. Historically it was a center of agriculture, notably groves of Valencia oranges and other citrus crops; petroleum extraction; transportation; and manufacturing.