I believe it officially came into being after WWII as part of the National Security Act in 1947.
The Marine Corps has been in existence as a separate service, though subordinate to the navy, since 10 Nov 1775. The subordinate part ended in about 1949, with the modern defense act which created the air farce and all, and the Commandant became a co-equal member of the JCS. We are still a department of the Navy, the MEN'S department. Though our spiritual and medical support come from Navy Chaplains, doctors and Corpsmen, the last of whom are Marines in everything but name.