Campbellton grew in Atascosa County, which was created from Bexar County in 1856 by the Sixth Legislature of Texas, and took its name from John Campbell, who came from Ireland to Texas as a boy with his parents. Campbell first came to Atascosa County around 1857 and began ranching. When the Civil War broke out, he joined the Confederacy and enlisted in Company I, 32nd Regiment, Texas Cavalry, serving as second lieutenant. After the Civil War, he returned to Atascosa County and started a ranch between Pleasanton and Oakville. He later opened a store that became a relay station for the mail route and established the first post office in 1874. In 1880, he donated land for a school to educate the community, and the school also served as a place of worship until the Sacred Heart of Jesus Catholic Church was built in 1897. Train service began in 1913, the population grew, and a new brick schoolhouse opened in 1915. During the late 1950s, oil was discovered in the area, though ranching remained the leading industry. Campbellton was home to John Files Tom and Joseph Daniel "Dan" Struve, who served as Texas legislators in 1873 and 1961, respectively. For years, State Highway 9 and later U.S. Hwy 281 brought business to the community until Interstate 37 between San Antonio and Corpus Christi diverted traffic away, but residents continued to gather in support of the volunteer fire department and Sacred Heart Catholic Church.