Following a semi-circular route through the southern half of the city above boulevards built along the path of the former Fermiers généraux wall of 1784–1860, it runs between Charles de Gaulle–Étoile in the west and Nation in the east.

However, the anticipated difficulties of operating such a long line resulted in a separation of the circle into two parts, a north circulaire and a south circulaire, the circle divided where it intersected with Line 1 (Étoile and Nation).

[1] As connecting the Paris railway stations was an objective for the Métro, an initial plan was to then run the southern circulaire from Place d'Italie to Gare d'Austerlitz, to Gare de Lyon, and from there operate along Line 1 to close the loop at Nation.

As a result of aerial bombardments during World War I, defensive measures were taken for the elevated rapid transit lines.

With Paris again subject to air attack, it was also desirable to separate the underground and elevated sections of Line 5.

Work on the length of the current line 6 was not particularly difficult, apart from land stabilisation around Denfert-Rochereau due to disused underground stone quarries, and the occasional sewer displacements.

The original bridge, built in 1878, was replaced with one made of metal supporting the railway viaduct above.

Originally finished in 1864, it was widened by 5.5 m (18 ft) in order to accommodate the Métro and is the only viaduct in the system made of stone.

A change in Line 6's operation occurred during the 1970s: Kléber station was expanded to four tracks with two island platforms, a rare arrangement in the Paris Métro, and converted to the line's control terminal, with Étoile acting as a simple turn-around stop.

During this time, a temporary yard was created with 810 m (2,660 ft) of track to facilitate vehicle movement.