Born in Montgomery County, Pennsylvania, Miles was the son of James Miles and Hannah Pugh. Miles enlisted in Isaac Wayne's company, part of the Pennsylvania militia during the French and Indian War. He was wounded at the Battle of Fort Ligonier; later commanding the garrison at that fort. He was discharged and then reenlisted in Thomas Lloyd's company as a sergeant and was promoted to captain-lieutenant for the expedition to Fort Duquesne. He accepted a commission as captain in 1760 and commanded troops on Presque Isle. After the war ended, he went into business as a wine merchant and married Catherine Wister, daughter of John Wister of Grumblethorpe, Philadelphia, PA. He also entered politics, and was elected to the house of assembly in 1772. He was an early advocate for American independ