The son of the squire François Fiacre Guinement, seigneur de Keralio and of Marguerite Rose Bodin, Auguste entered the military academy in the citadel at Metz on 15 June 1732, aged just 17.
He stayed in it until 8 February 1734, when he joined the régiment d'infanterie d'Anjou, in which his elder brother Felix François Guinement lost his life whilst serving as a lieutenant at the Siege of Philippsbourg on 18 July 1734.
He also fought at San Pietro before his regiment was sent to relieve the bataillon du Dauphin, then blockaded by Imperial forces at Secchia (18 September 1734).
He fought at Guastalla on 21 September, where one of his contemporaries from the Metz military school, lieutenant Charles de Pagès, was seriously wounded.
On 13 January 1741 he was made a captain and put in command of a company of the 2nd battalion of the régiment d'Anjou, fighting with them in the Bohemian theatre of the War of the Austrian Succession.