Willacy County was formed in 1911 from parts of Cameron and Hidalgo counties and originally included what is now Kenedy County; it was named for state senator John G. Willacy. Kenedy was split from Willacy in 1921, when the long-settled ranchers of the northern (Kenedy) part of the county sought to separate from the newly arrived farmers of the southern part.
The Bermuda onion was introduced to Willacy County in 1912. It grew well and slowly displaced ranchland in the southern part of the county, becoming the most important crop. For many years the town of Raymondville held an annual Onion Festival, using the tag line, “The Breath of a Nation.” In 1940, the first oil wells were sunk in the county’s Willamar Oil Field; today oil production is a major part of the local economy, although increasingly eclipsed by natural gas. Also in the 1940s, sorghum was introduced to the county, gradually displacing cotton and other crops. Cattle ranching remains a substantial economic activity as well.