Returned to military service in the United States Army Artillery School at Fort Sill in Oklahoma.

After practicing law for several years in the Rio Grande Valley, he was elected to the Texas House of Representatives, where he served from 1953 to 1965.

While in the state House, de la Garza was known for sponsoring a large amount of legislation in the fields of education and the environment.

In 1964, de la Garza, a strong supporter of U.S. President Lyndon B. Johnson, ran for the United States House of Representatives and won a seat in South Texas.

Upon his return home to Texas, he donated his entire Congressional archive to his alma mater, which had by then been renamed to the University of Texas–Pan American.