On 5 April 1811, with the Cabildo's approval, the city resolved to mark Argentina's first national day by building a pyramid in the Plaza de la Victoria. The choice of a pyramid was never explained; some think it echoed the obelisks on the Porte Saint-Denis in Paris. At the insistence of the builders the monument was made not of wood but of solid materials, including hundreds of fired bricks, and the foundation was poured on 6 April amid music and raucous celebration. It was inaugurated on schedule, even though construction was not actually finished, festooned with the banners of the city's regiments. The festivities ran four days and included dancing, raffles, and, tellingly for a revolution that spoke of liberty, the freeing of enslaved people. The first pyramid stood about thirteen metres tall and was, to save time, left hollow inside.

The structure visible today is largely the work of the painter and architect Prilidiano Pueyrredón, who reworked it in 1856, building the new monument directly over the old. He crowned it with a statue of Liberty by the French sculptor Joseph Dubourdieu, a robed female figure wearing the Phrygian cap, the soft conical hat that has signalled freedom since the Roman manumission of slaves and the French Revolution. From the ground to the peak of that cap the pyramid now rises 18.76 metres. A golden sun was set on its eastern face, turned to look toward the Casa Rosada, the pink presidential palace; the other three sides carry laurel crowns in relief. Four allegorical figures of Astronomy, Geography, Navigation, and Industry, long exiled to a distant plaza, were finally restored to the monument's corners in 2018.

The plaza changed around the pyramid. In 1883 the old arcade that had split the square in two was demolished, joining the halves into the modern Plaza de Mayo. Mayor Torcuato de Alvear wanted to raze the pyramid and build a more imposing monument, and he canvassed the country's elder statesmen. Former president Bartolomé Mitre saw no reason to keep it; Sarmiento rejected its later additions; only Nicolás Avellaneda argued for restoring and preserving it. The committee favoured demolition, but the national government preferred to leave things alone, and so it stood. In 1912, planners hoping to encase it in a vast new monument instead simply moved it. Between 12 and 20 November, the entire pyramid was jacked onto wheels and winched 63 metres east to the centre of the square. The grand monument meant to surround it was never built.

On 30 April 1977, fourteen women came to the Plaza de Mayo to demand word of their sons and daughters, seized and disappeared by the military dictatorship. Ordered by police to keep moving, they began instead to walk in a slow circle around the pyramid, white kerchiefs on their heads. From that act grew the Madres de Plaza de Mayo, who have circled this monument every Thursday since, in one of the longest sustained protests in the world. One of their founders, Azucena Villaflor, was abducted in December 1977 and murdered in a death flight over the river. Her remains were identified in 2005, and on 8 December that year her ashes were buried at the base of the pyramid, on the very spot where she had organized her first protest. The sidewalk around the monument is painted with the Mothers' kerchiefs. An obelisk built to mark a nation's birth now also marks its deepest wound, and the refusal to forget it.