Authorized in January of 1777, the 4th Light Dragoons were commanded by Irish born Stephen Moylan who had previously been the Quartermaster-General of the Continental Army. By the summer of 1777 the regiment had almost one-hundred and eighty dragoons in the field. The 4th Dragoons were present at the battles of Brandywine, Germantown, Monmouth, Whitemarsh, and the harsh winter at Valley Forge.

The regiment spent 1779 and early 1780 in Connecticut and New York, and was involved in countless small actions with the British and Loyalist horse in the vicious no-man's land in that theatre of the war. As the war progressed it became increasingly difficult to procure suitable horses due to the empty coffers of congress. In the fall of 1779 Washington instructed the four dragoon units to convert to "legionary corps", and recruit men to serve as attached light infantry. On January 1st, 1781, the 4th Dragoons became the 4th Legionary Corps, and signed fifty-five men of the Lancaster County, Pennsylvania militia into their light infantry company, thus making infantry an integral part of the then mounted regiment.

The 4th Legionary Corps held the post of honor on the right flank at Yorktown. After Yorktown, the Legion was sent south to reinforce Generals Green and Wayne, and participated in fighting British regulars and Creek Indians in Georgia. In the summer of 1782 the Legion returned with General Wayne to assist General Greene in the siege of Charleston, South Carolina.

During the closing months of 1782, the 4th suffered heavy casualties due to a plague that ravaged the army; the remaining men were brigaded into one troop of dragoons and one company of light infantry (under captains John Heard & Erasmus Gill, respectively).. With the British withdraw from Charleston, the reduced regiment marched home to Pennsylvania. In May of 1783 the 4th Legionary Corps arrived in Philadelphia. The Regiment was mustered out in June of that year "after being received with the ringing of bells by the joyous and gratified populous", a fitting end to a long and distinguished military career.