There's no way that, in the movie canon, John Wick was a marine. Come on, that's so lame. It's revealed in the second movie, I think, that John Wick was born in the USSR, trained from early childhood to be an assassin for the criminal syndicates that control the underworld. They don't need to say "he's badass because he was a marine." John Wick doesn't need that. Being a marine doesn't make the character more interesting, it makes him worse.

I looked it up and found a reddit post that claims one of his tattoos is a latin motto that is popular among marines, and also that the video game payday 2 claims he was one. I'm going to discount payday 2 because I doubt whoever wrote John Wick's blurb had access to whatever story bible the John Wick team has. As for the tatto, I mean, he is a soldier of fortune, but for the criminal underworld. It'd make sense for him to have a tattoo like that without ever having been in the military.

Also that reddit post, which speculates that John was discharged from the military for being mentally unstable, has this line: "Of course, John would never get to be a Marine if he had psychological issues before being in the army."

Anyway, here's my impression of Jarhead John being asked how he became the world's deadliest man:

"Hey John, is it true you incredible fighting ability is due to the fact that you were fostered by each of the world's most powerful crime syndicates, trained in their fighting styles, and then spent ten years with the secret, still extant Order of Assassins?"

"Oh, yeah, that was cool, but most of it came down to being yelled at by a guy in fatigues for a few months and then calling in airstrikes any time we heard a noise."