Born in Kassel of a bookseller and a merchant's daughter, Johann Ewald demonstrated an interest in warfare and military history at an early age. At the age of 16 he entered the Hessian service as a cadet and joined the infantry regiment Gilsa. He saw hard service during the Seven Years War followed by fourteen years of garrison duty. During his time between the wars, Ewald studied military science and published his first military treatise.

In 1774 Ewald was promoted to Captain and given command of the Lieb Jäger Korps. This was a major achievement for a commoner to reach this level of rank and position, and to be given command of such an elite unit simply amazed the nobles. Two short years later Ewald found himself preparing his unit for war once again, this time fighting along side the British in America.

Ewald sailed into New York on 14 October 1776 and commanded his first action with the Jägers on the 23rd of October. As with most of the Foot Jägers, they saw action in every campaign in the war. In larger actions Ewald's Jägers led the van of Cornwallis's column at Brandywine and Monmouth. During the opening days of the Southern Campaign in 1780, Ewald and his Jägers led the British Army from their landings on the South Carolina coast and supported the siege of Charleston. In May 1780 Ewald was ordered to return to New York and did not participate in the Southern Campaign after Charleston fell.

At wars end Ewald returned to Hesse-Kassel. In 1788, after being denied promotion twice because of his commoner birth, Ewald entered the Danish Army and appointed as a Lieutenant Colonel. In 1790 he was ennobled and thus became Johann Von Ewald. The son of a common bookkeeper, who at age 26 had lost his left eye, finished his 56 year military career as a Royal Danish Lieutenant General and the Commanding General of the Duchy of Holstein.

Ewald was a courageous and daring officer who earned the respect of both his superiors and subordinates. Ewald and the Jägers that served under him had gained such respect from the Americans they had fought against that while on parole after the surrender at Yorktown, Ewald visited West Point as the personal guest of General Henry Knox.

Beyond his military achievements on the battle fields, Ewald also achieved great respect from many military leaders for his writings. The most notable with regard to the Revolutionary War in American was his diary, which was well detailed and extremely accurate. Of the eight books written by Ewald, his 1785 "Treatise on Partisan Warfare" was praised by Frederick the Great of Prussia. "Treatise on the Service of Light Troops", published in 1790, was used by Major General Sir John Moore, who was credited as being the originator of modern light infantry training.