In 1755 young Ewing joined General Edward Braddock’sexpedition into western Pennsylvania, and in 1758 he served as a lieutenant in the Pennsylvania militia. On July 4, 1776, Ewing was commissioned a brigadier general in the Pennsylvania militia. Characterized by historian David Hackett Fischer as a "hard-driving Scotch-Irish border chieftain", Ewing commanded a brigade of five regiments at the time of George Washington's crossing of the Delaware Riveron the night of December 25–26, 1776, in a surprise attack on the Hessian forces in Trenton, New Jersey. Ewing's force, positioned directly across from Trenton, was unable to cross the river because of the ice. Although Ewing has sometimes been criticized by historians for failing to join Washington on the other side of the river, Fischer argues that no one could have crossed the river at that point that night. Washington did not blame General Ewing, writing that "the Quantity of Ice was so great, that tho' he did every thing in his power to effect it, he could not get over."
Political service
Ewing represented York County in the General Assembly from 1771 through 1776. In early 1776 he was one of the few Assemblymen calling for an immediate break with Great Britain. It was, in part, his enthusiasm for Independence that led to his general’s commission on July 4. Following Independence Ewing aligned himself with wealthy business interests, as a Republican and Anti-Constitutionalist, and later as a Federalist. In 1781 he won a seat on the Supreme Executive Council of Pennsylvania. On November 7, 1782, Ewing was elected Vice-President of Pennsylvania, earning thirty nine votes to the thirty four won by the incumbent, James Potter. He was unanimously reelected on November 6, 1783, and served until November 6, 1784, when he was succeeded by James Irvine. At one day short of two years, his vice-presidential term was one of the longest in the history of that short-lived office. In 1784 Ewing served a one-year term in the General Assembly. As Vice-President of Pennsylvania Ewing served as an ex officiomember of the Board of Trustees of the University of Pennsylvania. In 1783 he also served as a trustee of Dickinson College in Carlisle, Pennsylvania. In 1789 he chaired a committee seeking to improve navigation on the Susquehanna River. In 1795 he returned to elected office, as a Federalist member of the State Senate. James Ewing died at his home near York, Pennsylvania, in 1806.