Long before the community of San Diego was established, its location was a watering hole on the route from Mier, Mexico, to Goliad. Hence the name Mier Street, which runs north and south just west of the Padre Pedro Plaza.

Exemplifying Spanish town planning, St. Francis de Paula Catholic Church is located on the plaza. Visitors are invited to say a quiet prayer, join a service, or simply admire the sanctuary of the 1908 white-brick Gothic Revival church. The parish dates to 1867 when Father Claude Jaillet raised money to build a chapel. It served as the only public place of worship between Corpus Christi and the Rio Grande Valley.

On the south side of the plaza, the Duval County Museum occupies a Colonial Revival stucco house with a two-story balcony. The museum provides an introduction to local history with displays of Catholic garments and artifacts. An extensive collection of church records from Mier is held, including information on births, baptisms, marriages, and deaths dating to the 1700s.