Apparently you just need to get hired for ANY job on base and you are automatically a marine.
I know the term has been around for over 15 years if not longer.
When I was a young nail-eater napalm-pisser stationed at Quantico in 96-97, there was an article in the base paper about leadership training for "civilian marines". I wrote a letter to the editor railing against the term civilian marine. Within 24 hours of it being published, my name was mud on base. I got a shit ton of evil emails and was called to the CO's office. I didn't get in trouble, but was told in no uncertain terms that I was not to reply to any emails or respond to anyone that confronted me F2F. I did a lot of smiling and nodding for the next few months when people would come up to me at the PX and ask "Are you the Sgt 0699 that wrote the letter?" As I said in my letter, I'm in favor of hiring civilians to perform jobs that don't require leadership of Marines or fighting spirit, but I don't think they should be called Marine.