Stephan Moylan was born to a wealthy Catholic family in Cork, Ireland. His family sent him to be educated in France. Moylan then worked in Lisbon for three years before finally moving to Philadelphia.

Moylan joined the American army in 1775 and initially held a variety of staff positions. He started off as Muster-Master General before becoming secretary to George Washington. Promoted to colonel, he moved on to become Quartermaster General in the American army. He resigned from this final position in 1776.

Moylan then gained a field command. He became colonel of the 4th Continental Dragoons. In 1780, the 4th Continental Dragoons were sent south to join Lafayette's forces in Virginia. However, it is not certain if Moylan came with them. Moylan is, however, listed as being present at the conclusion of the Siege of Yorktown. He was breveted to brigadier general on November 3, 1783, the date he left the army.