A member of a series of Gaullist parties (UDR, RPR, UMP, LR), Barnier has served in several French cabinet positions under the governments from Édouard Balladur to François Fillon from 1993 to 2009.

In 1995, Jacques Chirac appointed him Minister for European Affairs,[12] a role in which he served until the defeat of the presidential majority in the 1997 legislative election.

[14] He subsequently served as Foreign Minister in Jean-Pierre Raffarin's government until June 2005 when Dominique de Villepin replaced him with Philippe Douste-Blazy.

[5] In 2016, the investigating judge Sabine Kheris requested that the case of Barnier, Dominique de Villepin and Michèle Alliot-Marie be referred to the Court of Justice of the Republic.

The former ministers were suspected of having allowed the exfiltration of the mercenaries responsible for the attack on the Bouaké penal camp during the 2004 Ivory Coast conflict, killing nine French soldiers.

[43][44] Following gains by opposition parties in the legislative elections called by President Emmanuel Macron in the summer of 2024, the prime minister, Gabriel Attal, resigned.

[47] Barnier's initial challenges as Prime Minister was forming a new government, passing the vote of confidence (with a minimum of 289 out of 577 votes) and submitting the 2025 budget by 1 October to parliament according to Reuters, further remarking that it would "be no easy task with the budget deficit already this year running billions of euros over target, leaving Barnier tough choices about calibrating spending cuts and tax rises" and risking the government's position in parliament.

"[44] Jean-Luc Mélenchon, the leader of the left-wing La France Insoumise, said that Macron had "stolen" the election by not appointing a prime minister from the New Popular Front[52] and called for protests against the new government.

[53] According to France's Interior Ministry, around 110,000 people took part in these protests, which were held in Paris,[54] Montauban,[55] Nice, Lille, Strasbourg and Montpellier, as well as in several rural areas.

[56] On 2 December 2024, Barnier invoked article 49.3 of the French Constitution to adopt the Social Security budget for 2025 without submitting it to a parliamentary vote.

[57] The decision happened after several last-ditch concessions to find a compromise failed, prompting both the New Popular Front and the National Rally to file motions of no confidence against his government.

As a candidate, he stated his intention in "putting a stop to non-European immigration for three to five years," during an interview with French weekly magazine Le Point.

[61] He proposed to "immediately stop regularizations, rigorously limit family reunification, reduce the reception of foreign students and the systematic execution of the double penalty".

[61] On economic issues, Barnier has been characterised as close to the neoliberal policies of Emmanuel Macron, including the reduction of taxes and business regulation.

As a presidential candidate, Barnier proposed cutting production taxes by €10 billion,[61] raising the retirement age from 62 to 65, lengthening the working week and tightening the conditions for access to social assistance.

[61] During his presidential campaign, many media organisations commented that Barnier sounded like a Eurosceptic and Brexit supporter, contradicting previous positions he had taken on the matter.

[67] In the same year, he was among the conservative politicians, including Jacques Chirac and François Fillon, who voted against reducing the age of consent for same-sex relationships to 15, the same as that for mixed-sex couples.

[61] Barnier has been a member of the Sustainability and Legacy Commission of the International Olympic Committee,[69] and of the board of trustees of Friends of Europe, a Brussels-based think tank.