As a young man Robert entered the army in 1626 and became an officer in Donald Mackay's regiment, serving first in Danish service and later in Swedish service where he highly distinguished himself during the Thirty Years' War on the continent, particularly during the Battle of Lutzen in 1632. According to 19th-century historian Alexander Mackenzie there were three Generals, eight Colonels, five Lieutenant-Colonels, eleven Majors and above thirty Captains, besides a large number of other soldiers all of the name Munro in the Swedish army during the Thirty Years' War. However, although there were numerous colonels and lower ranks bearing the name of Monro or Munro during the Thirty Years' War, none actually held the rank of general in any of the continental armies between 1618 and 1648. It was most likely this man recorded as a major in the regiment of Colonel Robert Monro in 1639 (it had reformed in 1637), just prior to the regiment's return to Scotland.