I think from the formation of the corps, but that would need to be verified. We have some marines here on the threads, perhaps by the time you see this, they may have clarified that for you.
For us the important thing is that this IS the fact. The very early drops last spring were all about laying out the special relationship that POTUS has with the marines and the national guard.
When I served (Enlisted) I often heard the Marine Corps referred to as "The President's own..." and thought it was because of the Marine Corps Band and White House detail.
I was unaware.
Check this out
http://www.freerepublic.com/focus/chat/3646521/posts?page=1743#1743
Marine Corps Manual - see page 1-3 & 1-4
http://www.marines.mil/Portals/59/Publications/MARINE%20CORPS%20MANUAL%20W%20CH%201-3.pdf
here’s the law - CHAP. CXXXII.-An Act for the better organization of the United States’ “marine
corps.”
https://www.loc.gov/law/help/statutes-at-large/23rd-congress/Session%201/c23s1ch132.pdf