Elected to the Legislature in 1791, he was part of the Diplomatic Committee and sat alongside the Girondins, who supported the policy of war against Austria.
Re-elected in September 1792 as Member of Parliament for the Haute-Garonne in the National Convention, he sat with La Plaine, remaining close to the Girondists.
[3] [4] At the trial of the king, he proposed "Death, but [...] I think it would be worthy of the Convention to consider whether it would be useful to policy to delay the execution" which was supported by twenty-six MPs.
On 2 June 1793, Paris sections took over the Convention, calling for administrative and political purges, a low fixed price for bread, and a limitation of the electoral franchise to sans-culottes (working class radicals) alone.
After that, Mailhe confined himself to the Legislation Committee, not reappearing in the National Convention until several weeks after the fall of Robespierre in July 1794.