Garnier-Pagès was an active participant in the antiroyalist uprising of 1830, but he did not formally enter politics until 1842, when he was elected to the Chamber of Deputies from the Eure region. In 1842–48 he sat with the republican left and devoted himself to financial and commercial questions.

In 1848, as the Orleanist regime of King Louis-Philippe began to topple, Garnier-Pagès emerged as a leader of the “banquets campaign,” a series of anti-regime political rallies. When Louis-Philippe abdicated, Garnier-Pagès became mayor of Paris and then minister of finance in a new republican government. Faced with a desperate financial situation, he enacted a series of stringent fiscal measures, including a surcharge on direct taxation, which caused great public dissatisfaction and brought about his defeat in a bid for election to the Legislative Assembly (1849). His Histoire de la révolution de 1848, 11 vol. (“History of the Revolution of 1848”), appeared in 1864.

During the Second Empire (1852–70) Garnier-Pagès remained in private life until 1864 and then became a member of the Corps Législatif. He opposed the Franco-German War (1870–71) but joined the republican Government of National Defense when Emperor Napoleon III abdicated in 1870. Once again, however, unpopularity cost him a seat in the legislature in 1871, and he retired from public life. He wrote an account of his experiences during the Second Empire, L'Opposition et l'empire, 2 vol. (1872; “The Opposition and the Empire”).