It has housed the relics of the Empress Saint Helena, mother of Constantine, since 1819, for which it remains a site of veneration in the Roman Catholic and Eastern Orthodox churches.

[3] As the population of the neighborhood grew, the church was rebuilt in 1319, and underwent several major renovations and modifications, notably in 1611, 1727, and 1780, when an underground chapel was added.

The Boulevard de Sébastopol was constructed behind the church by then-President and future Emperor Napoleon III, resulting in the demolition of three chapels and the apse.

Based on the medical findings, Monsignor Richard, Archbishop of Paris, announced that the “reliquary contained the almost-complete torso of Saint Helena”, that “the head was missing and the limbs compressed”, and that “the state of the body conserved in the reliquary of the Église Saint-Leu-Saint-Gilles corresponded to the descriptions known to the Bollandistes in the 18th century”.

The reliquary was subsequently placed on open display above and behind the high altar, at the feet of the large crucifix, suspended between two pillars of the apse.

On 17 March 2000 the relics of Saint Helena were transferred to the crypt of the order's knights, making them more easily accessible for veneration.

The apse of the church was considerably shortened in the 19th century and three chapels were removed to make space for the new boulevard built by Napoleon III.

The four-part Gothic rib vaults over the nave transfer the weight of the roof downward and outwards to rows of columns, which form arcades with pointed arches.

The interior is decorated like a formal salon, with murals painted by the 19th-century artist Jean-Louis Bezard (1799-1881) depicting "The Baptism of Christ" and "The Original Sin".

[11] The second chapel displays a work of 16th-century sculpture by Jean Bullant (1550-1578), who was better-known as an architect than as a sculptor; his famous buildings included the Château de Chenonceau in the Loire Valley, bridging the River Cher.

The disambulatory of the church displays an unusual art treasure; a retable with three bas-reliefs made of alabaster, depicting scenes from the life of Christ.

They are made in a small format, designed for display in private oratories and in the altar pieces and retables of churches.