Douglas was the son of the Scottish born minister, Robert Douglas and his wife Mary, born Robertson. The Reverend Douglas was a nephew of the Attorney General of Pennsylvania Samuel Douglas, and studied theology at the Theological Seminary at York, Pennsylvania, before being admitted to the ministry in the German Reformed Church. Henry Kyd Douglas grew up on Ferry Hill Place, on the opposite site of the Potomac River from Shepherdstown, then in Virginia, not far from Sharpsburg, Maryland. His home not being more than two miles from Harpers Ferry, he watched the storming of the engine house by the Marines that ended John Brown's raid, and later witnessed the treason trial that followed. Douglas graduated from Franklin &amp; Marshall College in Pennsylvania 1858, studied law in Lexington, Virginia, and was admitted to the bar at Charleston, then in Virginia, 1860. Having moved to St. Louis, he returned to Virginia at the outbreak of the civil war.