Guist'hau left Réunion for Nantes to study law there, and was elected the mayor of Nantes in 1908. He went on to become a deputy to the Assemblée nationale from 1910 to 1924. He was minister of Public Instruction and Fine Arts in 1912, minister of Commerce and Industry in 1913, and finally, minister of the Navy in 1921. As minister of the Navy, he oversaw the reconstruction of the French Navy, after the ravages of the First World War.