He first entered military service in 1793 aged 14,[1] attached to the staff of General Cambray during the war in the Vendée.

[2] On 30 October 1795, he joined the 7th Chasseurs a Cheval as a Second Lieutenant, became aide-de-camp to General Songis on 10 December 1795, and spent the next two years serving in the Army of the North.

[4] In 1805, Merlin's regiment was recalled to the Grande Armée and he served in it for the next three years, fighting at the battles of Austerlitz, Jena and Friedland.

He fought at Leipzig and Hanau,[3] and was publicly praised by Napoleon who made him second-in-command of the 4th Guards of Honour.

The two men boarded the American vessel Alice in Antwerp and set sail for the United States, but were shipwrecked near Vlissingen in the mouth of the Scheldt on 24 February 1816.

[1] Returning to duty under the new regime in 1830, Merlin was made a Commander of the Legion of Honour on 21 March 1831 and added to the general staff.

Leading a brigade in the reserve cavalry division of the Army of the North during the Ten Days' Campaign, he was present at the Siege of Antwerp.

[2] He was promoted to lieutenant general on 30 September 1832 and held a succession of military posts culminating with the command of the 18th division on 7 June 1834.

[1] He became a Grand Officer of the Legion of Honor on 30 January 1837, a Count (on the death of his father) on 21 December 1838, and a Peer of France on 7 November 1839.