During the French and Indian War, he shared command of the frontier with George Washington. He also served in the Revolutionary War, earning promotions for his actions at the Battle of Trenton.

After serious errors of judgment at the 1778 Battle of Brandywine ended his military career, Stephen returned to work in land development, medicine, and politics. He helped convince Virginia to approve the U.S. Constitution.

Stephen played a big role in creating Berkeley County in 1772 and helped start the town of Martinsburg in 1778. He planned the town and named it after Col. T. B. Martin. Stephen also served as the county's first sheriff of Berkeley County.