The Northampton star spent six weeks out of the England picture over the pre-season training run, as boss Steve Borthwick rated him as fourth choice scrum-half.
While rivals Jack Van Poortvliet, Danny Care and Ben Youngs were grafting away in England’s World Cup camp, Mitchell even spent one week on holiday in Croatia.
Even when Van Poortvliet suffered an ankle injury and Mitchell was drafted back into the squad on August 14, the 26-year-old still remained a long-shot to leapfrog both Youngs and Care.
Now, though, the strike running nine has done exactly that, and will line up alongside George Ford in England’s half-backs in Saturday’s Pool D opener against the Pumas at Stade Velodrome.
“For me it has been a bit of a rollercoaster, hasn’t it?” said Mitchell.
“Obviously out of the squad and now back in and getting a shot tomorrow.
“I am just massively excited at getting an opportunity to play and put my hand up. I am looking forward to that.
“It has been a bit weird but; I’m excited.”
If Mitchell’s World Cup hopes were dashed then restored across a turbulent summer, now he has the chance to realise his lifelong ambition.
Asked to sum up the magnitude of his World Cup debut, Mitchell said: “It’s massive. As a kid it’s the thing to dream of, playing at the top of the game.
“So, to start the first World Cup match is a massive honour. My family are massively proud of me.
“So, really looking forward to it and hopefully we can get a result.”
When Borthwick told Mitchell he was initially cut from the World Cup squad, the England boss urged the Saints man to stay sharp in case of injury.
Mitchell kept himself alert, and now is glad he did.
“It was a pretty positive conversation, it was ‘you’re not far off’,” said Mitchell.
“There was a lot of competition in my position so he just wanted me to go back to my club, still work on my game and still stay fit, because anything can happen, and be ready.
“That’s the conversation I had, so that’s what I did. I stayed fit, and stayed ready then got called to come in. I trained hard and tried to put my hand up again.”
Mitchell will seize the scrum-half shirt with the aim to super-charge England’s speed of attack.
“I’ve got to try to control the game, get the team to tick and when I can imprint some tempo into the side, I’ll try to do that, and again, stick to the gameplan.
“We know we’re a good side when we play well and stick to our style of play.
“The main thing is to do that and bring our X-factor alive after that, if you can.”