John Marston VI was born on August 3, 1884, in Somerset County, Pennsylvania, to a family with a long military tradition.

His great-great-great-grandfather John Marston I (1715–1786) was a captain with the 3rd Massachusetts Bay Artillery and participated in the Siege of Louisburg in the War of the Austrian Succession.

During the battle, he commanded a small detachment of Benét–Mercié machine guns and covered the advance of Major Smedley Butler and his men.

After his return to the United States, he was appointed commanding officer of Marine detachment within the Naval Academy at Annapolis, Maryland.

Following two years of service there, Marston was promoted to the rank of major on July 22, 1920, and transferred to the Marine Barracks Quantico, Virginia.

[2][3] With the increasing threat of German occupation of Iceland in May 1940, the British government sent Royal Marines under the command of Colonel Robert Sturges to prevent Germany from occupying strategic islands in the Atlantic Ocean.

Because of the critical need for troops on other battlefields, Great Britain requested that the United States occupy Iceland in June 1941, which was approved.

The 1st Provisional Marine Brigade was reactivated under the command of Brigadier General Marston and sailed on June 22 for Iceland from Charleston, South Carolina, via Argentia, Newfoundland.

[4] A battalion of Marines was relieved and left Iceland on January 31, 1942, but it was not until March 4 that the rest of the brigade began embarking for its departure.

[4] On March 8, Marston relocated his command post from shore to the USS McCawley, restoring the brigade to Navy jurisdiction.