Being destined for the bar, young Marshall began his legal studies at the age of eighteen, but in two years they were interrupted by the troubles with Great Britain, which terminated in open hostilities.  A volunteer company was raised in the neighborhood, and John Marshall promptly attached himself to it.  He took a prominent part in the questions of the day, and expressed himself boldly in favor of resistance.  In 1775 Patrick Henry made his memorable appeal for volunteers to drive the Loyalist Governor, Lord Dunmore, out of Virginia.  Three companies were immediately organized in Marshall’s neighborhood.  Among these were the famous “Culpepper Minute Men.”  Marshall’s father was elected major of the regiment, and he himself was chosen a lieutenant in the Minute Men.  The force at once hastened to the lower counties, and bore a conspicuous part in the battle of Great Bridge.  In July, 1776, Marshall’s company was assigned to the Eleventh Virginia Regiment of the Continental Army, and sent North.  In May, 1777, he was made captain of his company.  He participated in the fight at Iron Hill, and in the battles of Germantown, Brandywine, and Monmouth, and shared the sufferings of the army at the memorable encampment of Valley Forge.  Until the close of 1779 he was constantly in active service.  He was always patient, cheerful, and hopeful.  In the severest hardships to which the army was exposed his spirits never sank.  One of his comrades said that he did more than any other man to keep alive the hopes of the army during the terrible winter at Valley Forge, and another has declared that “the officers of the Virginia line appeared to idolize him.”  His conduct attracted the attention of Washington, who conceived a warm friendship for him, and Marshall, on his part, returned the friendship of his chief with a feeling almost of worship.  Washington frequently appointed him deputy judge advocate during the winter.

At the close of 1779 he went to Virginia to take command of a new corps which the Legislature was about to raise.  The project remaining under discussion for some months, he passed the time in attendance upon a course of lectures on law, delivered by George Wythe, and a course of lectures on natural philosophy, delivered by the Rev. Dr. Madison, afterward Bishop of Virginia, at William and Mary College, in Williamsburg.  The next summer he received his license to practice law.  Meanwhile, the project for raising troops had taken the shape of a definite failure, and he now set out to rejoin the army.  Too poor to pay his passage to the North, he walked the entire distance from Williamsburg, Virginia, to Philadelphia, upon reaching which city he was so travel-worn and shabby in appearance, that the landlord of the hotel at which he wished to stop refused him admittance.  He joined the army in due time, and remained with it until the spring of 1781, when he resigned his commission, a few months before the close of the war.