26 January – The French government and the Île-de-France Region merge their transit projects and sign an agreement that forms the Grand Paris Express.

20 March – The Grand Paris Express breaks ground at Aulnay-sous-Bois. Several thousand geotechnical probes will be required to accurately analyse the type of soil the metro will run through.

19 November – Société des grands projets attends the COP21 to introduce its role and unveil its environmental commitments while building and operating the new metro.

June – One year after preliminary work begins, the first civil engineering works for the Grand Paris Express start at the future site of the Fort d’Issy-Vanves-Clamart station.

30 March – Line 18 is designated a public good along with the entire Grand Paris Express system.

6 February – Underground works begin. To mark the event, thousands of people come to see the first Grand Paris Express tunnelling machine in Champigny-sur-Marne.
 

18 December – The Grand Paris Express's first ‘Tandems’ are unveiled. The group of artists and architects will be creating an urban art collection for the new metro's stations unlike any other in the world.

14 April – The first load of rubble is hauled away by barge straight from a worksite in Aubervilliers. Over 15% of the earth excavated for the Grand Paris Express is transported by river and rail.

9 May – The La Courneuve Six-Route hub project is approved, the first in a long series. Each Grand Paris Express station is an opportunity to redesign public spaces and develop surrounding modes of transport.

29 October – The first Grand Paris Express tracks are lain at the Noisy-Champs station sidings.

November – Société des grands projets begins planting trees at La Plaine de Pierrelaye-Bessancourt to offset the impacts of construction. The forest located 30 km outside Paris is 50% larger than the Bois de Boulogne.

November – Maiden voyage of the test train for the rolling stock that will run on Lines 15, 16, and 17.

April – The first ultra-low-carbon concrete voussoirs are installed and tested on Line 18. Société des grands projets spearheaded this innovation to reduce CO2 emissions during construction.

24 June - Line 14, extended to Orly airport, welcomes the first passengers.