He was a professed Catholic, and therefore was denied a career in government. A career in the army was the obvious choice. Overcoming the challenges posed by his faith, he had a distinguished career in the Dutch Army, participating in the War of the Grand Alliance and the War of the Spanish Succession. In 1675 he was the Colonel of one of the Dutch-Scottish regiments. He became Major General in 1683, Lieutenant-General in 1692 and General in 1704. In 1689 he commanded troops at the Battle of Walcourt. In 1703 he rescued a desperate situation at the Battle of Ekeren. Slangenburg's relations with the Duke of Marlborough were not good, which in 1705 led to his removal from his post and replacement by Ouwerkerk, who had a far better personal relationship with Marlborough.