In 1784, Macdonald joined the Irish Legion, raised to support the revolutionary party in the Dutch Republic against the Kingdom of Prussia and was made lieutenant on 1 April 1785. After it was disbanded, he received a commission in Dillon's Regiment. At the start of the French Revolution, the regiment of Dillon remained loyal to the King, except for Macdonald, who was in love with Mlle Jacob, whose father was an enthusiastic revolutionary. After his marriage on May 5, 1791, on 17 August 1792, he was promoted to captain, and on 29 August 1792 he was appointed "aide-de-camp" to General Charles François Dumouriez. He distinguished himself at the Battle of Jemappes, and was promoted to lieutenant colonel on 12 November 1792 and then colonel on 8 March 1793.