Barangay San Pedro was formerly known as Km.47 of the Daang Maharlika (National Highway). It was created on February 8, 1953 by the Municipal Council of Mainit and was named in honor of its sponsor no other than the Municipal Councilor member Pedro A. Umipig, who later becomes the third elected Municipal Mayor of Alegria. This place was founded by the early 12 settlers led by Zacarias Daleon, Engracio Gamas, Magdalina Mosquito, and Jose Orquina, among others.
The first Teniente del Barrio (or synonymous to Barangay Captain of today) was Eleuterio Timcang. His successors among others were Magdalina Mosquito, Zosimo Reyes, Teodolu Abarrico, Salvador Patagan, Justimo Samoya, Eleuterio Potante, Albino Edillo, Carlos E. Babaison, Sr.
Due to the appreciation of its population growth rate every year since the beginning of its creation as a barrio, the barangay is now considered as an urban barangay. As a matter of fact the barangay at the present has lively and existing business transactions. Mostly, the migrants from the Provinces of Negros, Bohol and Leyte gave favorable contribution to build the contemporary socio-economic condition of Barangay San Pedro.
Barangay San Pedro is now a progressive barangay from his being a very rural place. The barangay created the Sitio of Sinaygan, Magtiaco and Camboayon. Its literacy rate improved due to the presence of schools like Day Care Center and Elementary School. Schistosomiasis which was the common disease few years ago is now drifted into low level after the establishment of functional Health Center in the Municipality.