The presence of man in the area of Cuyo dates back to 8500 years ago; that is to say, 6500 years B.C., in the caves of Inti Huasi (from the "quechua" language, it means Casa del Sol), which witnessed the presence of tribal groups in the territory of San Luis.
Apparently, these groups disappeared only to reappear 300 years later, in 5,900 B.C., when the area had acquired its present features, to devote themselves to hunting and fruit picking, such as the algarrobo.
As from that moment, life in the territory of San Luis changed and was reduced to the side of the mountain range. The most favorable areas for hunting in the summer were the high lands, whereas fruit picking was perfomed on the littoral of the large rivers and the low lands, particularly, the Conlara Valley.
When in 2000 B.C. aborigine groups bringing the grounds for agriculture and cattle-raising entered the area coming from Southern Perú, the human groups could no longer make their temporary moves according to the weather in each season and they started their search for intermediate areas for their settlement.
As from the year 700 AD, village groups started to be generated in the small protected valleys which allowed cultivation in their lower and more humid parts, whereas the funnel-like walls of those valleys were ideal for cattle pasture. The high pampas and the uncultivated areas continued contributing with their dose of hunting and picking of various fruit. The conditions of the mountain range could not permit a pattern other than that of the scattered village.
Nowadays, many of those village groups have turned into settlements of different degrees of importance that today inhabit the Conlara Valley.
Towards 1830, San Luis was undergoing the attacks of Indian raids, which kept advancing towards the Norht, fighting everything on the way.
At the end of 1832, Facundo Quiroga is appointed governor. The well-known Tigre de los Llanos (Tiger of the Plains) resolves to protect the Punta de los Venados (name held by the province in its origins).
Towards 1850, the situation had not presented significant changes. However, there was a feeling of expansion. The realization of this expansion started towards the South, with governor Justo Daract, who founded the Fuerte Constitucional on December 1st., 1856, el Fuerte Constitucional (Constitutional Fort), in the surroundings of the San Lorenzo del Chañar fort, by the Quinto River, acting as a civilization advancement in an area dominated by the pampas indians.
In 1861, when it turned into a village, it adopted the name of Mercedes, invocating the protection of Nuestra Señora de las Mercedes (Our Lady of Mercy).
It was declared a city in 1896 and it is at present an outstanding commercial, industrial and road center.