The regidor served as an official on the ayuntamiento, the Spanish town council, which functioned as the primary component of Spanish municipal administration. The council, known also as the cabildo, was composed of regidors, the alcalde, and assorted other city officials. The number of regidors ranged from four to twelve, depending on the size of the municipality. Spanish law provided for the annual election of regidors, but elections were often not held. Although some regidors were popularly elected, the offices were often sold to the highest bidder, and the proceeds went to the royal government, not to the local municipality. Regulations required regidors to be landholders in the town in which they served. They could not involve themselves in areas where there might be a conflict of interest.
The duties of regidors did not appear formally stated, for tradition played a large role in their activities. In general they acted as town councilmen. One of their prime functions was to select the alcalde ordinario, who not only served on the ayuntamiento, but also functioned as the town's chief executive and local judge. Although the regidors were subordinate in authority to the alcalde, they shared equal votes on the council. The members raised taxes and supervised local agriculture. They also looked after police and security matters. Additionally, the regidors performed a judicial function, for the ayuntamiento served as the first court of appeal of an alcalde's decision. The oldest regidor served as chief assistant to the alcalde and acted in his place during the alcalde's absence.
A royal decree of 1793 provided suggestions for the division of responsibilities between the regidors. Under this plan, each was given authority over a distinct aspect of public life, such as prisons or weights and measures. In the villa of San Fernando de Béxar the regidors divided their administrative duties in accord with the decree. The Spanish Constitution of 1812 further formalized the officeholding requirements and set a term of two years, but did not fundamentally change the position. During the latter part of the colonial era and under Mexico, regidors performed their duties in the traditional manner, relatively untouched by political changes in the central government.