Castaner was Government Spokesperson under Prime Minister Édouard Philippe in 2017 and Secretary of State for Relations with Parliament from 2017 to 2018.

His tenure, which was marked by the yellow vests movement, was heavily criticised for its scenes of police brutality,[4][5][6] as well as a series of controversial public statements he made.

[7][8] While France was battling the COVID-19 pandemic and demonstrations had been banned, he allowed a Black Lives Matter protest to take place, attracting further criticism.

[12] He was once again reelected Mayor of Forcalquier on 23 March 2014 by 22 votes, standing against Sébastien Ginet of the Union for a Popular Movement (UMP).

Castaner was named on 5 February 2015 as lead candidate in the upcoming Provence-Alpes-Côte d'Azur regional election by members of the Socialist Party with 55% of the vote, ahead of Patrick Allemand (31%) and Elsa di Méo (14%).

A member of the National Assembly's Finance Committee, in July 2012 Castaner was appointed Special Rapporteur of Work and Employment Budgets.

[17][18][19][20][21] Castaner justified his joining with Emmanuel Macron by explaining that in politics one must be "at the right place at the right time, without necessarily knowing where you will end up".

[23][24] On 25 October 2017, a few days after Emmanuel Macron gave him his support, Castaner he declared his candidacy for the chairmanship of the La République En Marche party on RTL.

[26][27][28] In June 2020, in the aftermath of the yellow vests movement and the death of Cédric Chouviat in early 2020, he announced that chokeholds would no longer be taught in police academies in France as an arrest technique.

[29] On 9 March 2019, after a day of yellow vests demonstrations, Castaner, a married man, was photographed in a nightclub in Paris with an unknown young woman.

Videos and testimonies published the next day revealed that what he presented as an attack was in fact demonstrators trying to escape from police, who charged to disperse rioters.

[32] In the aftermath of the Paris police headquarters stabbing in October 2019, Castaner was criticised for the public statements he had made regarding the motive of the attacker.

Castaner had claimed that there were no warning signs prior to the attack in the suspect's behavior, while records dating back to 2015 documented several alerts and reports concerning a possible Islamic radicalisation.

[8] In June 2020, Castaner announced a Black Lives Matter demonstration in Paris would be authorised to proceed despite a decree signed by the Prime Minister banning demonstrations as a result of the COVID-19 pandemic in France and although he had stated the week prior "gatherings were prohibited" in order to slow the spread of the virus.

BFM TV published as a headline: "Christophe Castaner replaced by Gérald Darmanin after two years of controversies".

On 10 September 2020, Castaner succeeded Gilles Le Gendre as president of the La République En Marche group in the National Assembly.

[34] Castaner was defeated in the second round of the 2022 French legislative election by Léo Walter of La France Insoumise and subsequently lost the seat he held since 2012.

[35] In October 2020, Castaner was one of 48 LREM members who voted in support of a bill introduced by the Ecology Democracy Solidarity parliamentary group that would extend the legal deadline for abortion from 12 to 14 weeks.