The service runs every day of the week, and the line uses MF 67 series trains, the network's standard since the early 1970s.

They operate the line today and have plans to extend it south as far as the town of Issy-les-Moulineaux and north to La Plaine in Saint-Denis.

Some stations are decorated thematically: Assemblée Nationale has murals explaining the intricacies of the lower house of the French Parliament, while the tiling at Concorde represents an extract from the Declaration of the Rights of Man and of the Citizen (1789).

Engineer Jean-Baptiste Berlier proposed to the City of Paris to finance and build a new line linking the areas of Montmartre and Gare Saint-Lazare in the north with Montparnasse and Vaugirard in the south.

This method allowed the straightest line possible to be built, without passing underneath buildings and free from interference with underground sewers.

[4] The link between two important but distant urban centres guaranteed heavy traffic for the line and it would also directly compete with the CMP and tramway companies, threatening their monopoly across the city.

[5] The City was wary of inciting new demands to license other lines, and of eventually provoking industrial disorder, something already experienced on the tramway network.

The law announced the creation "of [] public utility, of local interest, this establishment, in Paris, of a railroad, electrically powered, dedicated to the transport of passengers and their hand-luggage, from Montmartre to Montparnasse".

[2] The boggy undersoil of Paris made it impossible for the engineers to follow their initial concept of deeply excavated, metal-lined tunnels: digging deeper to more stable ground would raise costs.

On the opening day, public representatives rode from Notre-Dame-de-Lorette to Porte de Versailles, and returned to Gare Saint-Lazare for a buffet held in the rotunda.

The route passed underneath the hill of Montmartre, which had long been quarried and mined for gypsum to make plaster of paris.

[15] On 12 July 1928, the Seine general council decided to extend the Métropolitain by 1.5 km (0.93 mi), taking it beyond the limit of Paris so that line A would serve the town of Issy-les-Moulineaux with two new stations.

[18] On the night of 20 April 1944, during the Liberation of Paris, the freight station of Porte de la Chapelle and the RATP central workshop on rue Championnet were bombarded.

Beginning at Issy-les-Moulineaux, south-west of Paris, with a three way tunnel underneath l'avenue Victor-Cresson, the terminus is at Mairie d'Issy, and has only two tracks.

It runs north-east, entering Paris at Porte de Versailles, a major station with three tracks, one of which gives access to the workshop at Vaugirard.

After Falguière, the line veers back south-east in a 150 m (490 ft) radius bend underneath the Boulevard du Montparnasse.

[22] In preparation for the ascent of the Montmartre hill, the line veers sharply north in two curves of 150 m (490 ft) radius, putting the tunnel under Rue Notre-Dame-de-Lorette.

[23] Between Abbesses and Lamarck-Caulaincourt stations, the tunnel crosses Montmartre at a maximum depth of 63 m (207 ft), close to the Basilica of the Sacré Cœur, making Line 12 the network's deepest.

The tunnel runs underneath the railways departing from Gare du Nord, then slants northwards in a 50 m (160 ft) radius curve into Marx Dormoy station, in the Goutte d'Or neighbourhood.

The line continues down a slope of 2.6 per cent, with new bends, before arriving at Porte de la Chapelle station, on the northern edge of Paris.

[24] Beyond the depot, a new tunnel was constructed for the first phase of the extension towards Aubervillers, passing underneath the ring road, and exiting the northern fringe of Paris for the first time.

After this, the line continues via Aimé Césaire to Mairie d'Aubervilliers(From fr:Ligne 12 du métro de Paris).

[25] One of the notable elements is the Saint-Lazare station in which architect Lucien Bechmann designed a rotunda for the tickets and transfer room.

[27] In the stations, the supporting walls are vertical and not vaulted, and the ceramic tiles carry the customary "NS" logo of the company.

Five stations have unique décor, each based around a single theme: Abbesses, Concorde, Assemblée nationale, Montparnasse – Bienvenüe, and Pasteur.

The rolling stock of Line 12 is maintained at the Vaugirard workshop,[45][46] situated underground in the 15th arrondissement of Paris between the rues Croix-Nivert, Desnouettes and Lecourbe, and Lycée Louis-Armand.

[51] There have been two accidents on the line: On 23 April 1930, a collision was caused by human error close to the Porte de Versailles station.

The train's automatic control function was broken, and the driver was unused to driving manually and arrived too fast into the steep decline before Notre-Dame-de-Lorette station.

[46][53] The investigation concluded that a specific emergency signal be installed on the approach to the station, and that drivers preserve their skills by not habitually driving with the auto-pilot function.

The project was included in phases 2 or 3 of the Île-de-France regional master plan (SDRIF) adopted on 25 September 2008, with an expected start in 2014 or 2020.