Rafael de Aguiar founded the town on April 14, 1748, naming it for Saint Nicholas of Bari, who remains its patron. For a century it was a quiet river settlement near the borders of three provinces, and that position made it the natural stage for Argentina's defining quarrel: whether the country should be a loose federation of provinces or be governed from Buenos Aires. On May 31, 1852, thirteen provinces met here and signed the Acuerdo de San Nicolás, ratifying the Federal Pact and calling for the constitutional assembly that Justo José de Urquiza had championed. The agreement set Argentina on the path to its 1853 constitution. The historic building where it was signed survives as a museum, an unassuming colonial structure that once held the men deciding what kind of country they would build.

San Nicolás earned a second name in the twentieth century: Ciudad del Acero, the City of Steel. In 1947 the state created SOMISA, the company that would build Argentina's first integrated steelworks here on the Paraná. President Arturo Frondizi inaugurated the plant in 1960, and it poured its first ribbon of molten steel the following year. At its height the works employed more than eleven thousand people and made San Nicolás synonymous with heavy industry. Privatization under Carlos Menem in 1992 cut the workforce hard before the plant passed to the Techint group; today it operates as Ternium Argentina. The city's deep-water port still handles large cargo ships, and the blast furnaces still glow at night, a working monument to an industrial dream.

The apparitions began on September 25, 1983, when Gladys Quiroga de Motta reported seeing the Virgin while praying the rosary. According to her account, a ray of light marked the spot for a future sanctuary, and Mary told her plainly: I want to be on the banks of the Paraná. Bishop Domingo Salvador Castagna ordered a shrine built; construction started in 1987, and the sanctuary was consecrated in 1990. For decades the Church withheld judgment. Then, in a decree read at Mass on May 22, 2016, Bishop Héctor Cardelli declared the events of supernatural character and worthy of belief, a rare and significant ruling. The decision transformed San Nicolás into one of Latin America's major Marian destinations, the Ciudad de María, City of Mary, drawing the long columns of pilgrims that now define each May.

Beyond steel and devotion, San Nicolás keeps the texture of a complete Argentine city of roughly 133,000 people. The Rafael de Aguiar Municipal Theatre, opened in 1908, was designed as a smaller echo of the famous Teatro Colón in Buenos Aires, bringing grand-opera ambition to the provinces. The city raised its share of footballers too, including the legendary Omar Sívori, who would star for Juventus and the Argentine national side. Sitting within the industrial corridor that runs from Greater Rosario down to La Plata, San Nicolás lives at the meeting point of currents that rarely coexist so openly: river and railway, faith and furnace, the pilgrim's slow walk and the freighter's heavy wake.