The origin of the Municipality goes back to the end of century XVIII, when the Portuguese crown distributed lands on the banks of the Lagoon of the Ducks to military that stood out in the wars against the Spaniards. The owners built chapels in devotion to their favorite saints. In 1807, the residents of Fazenda do Boqueirão built the chapel of Nossa Senhora da Conceição, around which the village was developed which is the cradle of the municipality. In 1830, the village of Fazenda do Boqueirão was elevated to the Parish, by Dom Pedro I, being dismembered from the village of Rio Grande and incorporated into the village of São Francisco de Paula, now Pelotas.

In 1850, Colonel José Antonio de Oliveira Guimarães donated part of the farm land to a new settlement and in 1858 signed a contract with the Prussian Jacob Rheingantz that gave rise to the German colonization, predominantly Pomeranian in the region. The small port located at the mouth of Arroio São Lourenço, which had once served the squadron commanded by Giuseppe Garibaldi during the Farroupilha Revolution, became one of the most important ports of merchant sailboats in southern Brazil, contributing to the progress of the colony that was large Producer of potatoes during the 19th century and part of the 20th century. The house where Rheingantz installed the administration of the colony and its residence is preserved and integrated to the architectural patrimony of the Municipality.

A stage of many battles in the 19th century, due to the Farroupilha Revolution, São Lourenço do Sul houses important passages of history about the formation of the State of Rio Grande do Sul. The Fazenda do Sobrado, located on the banks of the Lagoa dos Patos and which served as a refuge For Giuseppe Garibaldi, besides being used as headquarters by Bento Gonçalves during the battles against the Imperial Army, is a testimonial proof of these facts.

These events are kept alive in the memory of the city and preserved for posterity. It is in this spirit that St. Lawrence writes his story, with much respect for what has passed and eager for new conquests.

Although the Parish of Boqueirão was elevated to the condition of village and emancipated of Pelotas in 26/4/1884, the seat of the new municipality was transferred in 15/2/1890 to São Lourenço, that in 31/3/1938 happened to Be city.