In 1804, Napoleon I appointed him as governor general of the French colony in Saint-Domingue and Guadeloupe, following the suppression of a widespread slave insurrection.

Before he could be exonerated by a court, the First Empire fell; with the Bourbon Restoration, he retained his honors, and received command of the III Corps, in Marseille.

He was commissioned as a lieutenant of infantry in the 1st Battalion of Volunteers of the Orne on the 24 September 1791, and as a captain on 22 March 1792, and 5 May 1793 he became an aide-de-camp of General Barthel's Army of the North.

The Duke of York laid siege to Dunkirk and blockaded the town of Bergues, on the Belgian border, which had insufficient garrison to fend off the British.

Ernouf assembled a force of a thousand men and joined Jean Nicolas Houchard; together they marched to the relief of Dunkirk.

[1] It was also by his advice that the commander-in-chief, Jean-Baptiste Jourdan, discovered Josias, Prince of Coburg's unfortunate position behind the Wattignies forest, compelled him to retreat across the Sambre and subsequently lifted the siege of Maubeuge: Ernouf's part in this action, the Battle of Hondschoote, earned him his promotion to major general on 23 frimaire an II (13 December 1793).

Ernouf was with the Army of Observation when it crossed the Rhine river, in what British historians have called a violation of the Treaty of Campo Formio,[2] resulting in the War of the Second Coalition.

[10] Attacked on three sides at the end of January 1810, Ernouff's force mounted a spirited, although short defense and capitulated on 6 February 1810, after which he was transported to Britain.

The mere news of Napoleon's escape from Elba and the defection of some of the troops caused Charles, Duke of Angoulême, to panic and capitulate.

On 11 November 1816, Enrouf received command of the III Division, located at Metz, which was occupied by Allied troops as a condition of the Second Treaty of Paris; his role was to maintain harmony between residents and the foreign soldiers.

[1] Elected by the Moselle, in 1816, he obtained in 1818 permission to sit in the Chamber of Deputies, and left the command of the III Division when he became eligible for retirement on 22 July 1822.

[11] Ernouf's son, Gaspard Augustin (8 December 1777 – 25 October 1848), was also a military commander during the French Revolutionary and Napoleonic Wars.