At the time the local community was granted its first post office in April 1907 it was known as Diaz. Because another town in Texas had previously been awarded the name Diaz by postal officials, the name of this post office was changed to San Benito later that year. Colonel Samuel A. Robertson served as first Postmaster and Emma (Purvis) Agar became the first of several women postal clerks to serve in San Benito. Municipal delivery began in 1925 along two routes.
The post office operated from several locations in San Benito until relocating in 1933 to an impressive Federal office building newly constructed here on park land donated by the city. The post office opened on April 16, 1933, the day after an estimated 3,000 people attended an open house to inspect the building. Also housed in the Federal building at the time of its opening were offices of the International Boundary Commission, the U. S. Departments of Agriculture and Immigration, and the U. S. Civil Service Commission. The post office served as a social gathering place for many people in the community. San Benito Post Office currently serves San Benito and the surrounding towns of Los Indios, Blue Town, Arroyo City, Lozano, Las Yescas, Laurles, La Paloma, El Ranchito, and Rio Hondo.