Son of a clerk at the Ministry of War, Alexandre Méchin was a partisan of the French Revolution, enlisting among the Jacobins in 1790 before approaching the Girondins, which earned him the ban on May 31, 1793.

He returned to France after 9 Thermidor and followed Louis-Marie Stanislas Fréron in his mission in the South in year III (October 1795).

He left a Precise of my trip and my mission to Italy in the years 1798 and 1799 and account of the events which took place in Viterbo from the November 27, 1798 until the December 28 next (1808).

He was revoked during the first Restoration in 1814, and, according to Laurent Esnault (Mémoires sur Caen, year 1814), "was not regretted; since the grain insurrection he was generally hated".

From 1823, whereas the expedition of Spain risks, according to him, to bring about the fall of the Bourbons of the elder branch, he evoked with Stanislas de Girardin the idea to entrust to the Duke of Orleans the general lieutenant of the kingdom, "by virtue of a wish made by part of the chambers and a certain order of soldiers, officials and citizens".