The Northampton scrum-half was omitted from England’s initial 33-man World Cup squad, but was drafted straight back in after Jack van Poortvliet’s ankle injury.
Now the 26-year-old has leapfrogged veteran duo Danny Care and Ben Youngs, to start England’s pivotal Pool D opener at the Stade Velodrome.
Mitchell offers rapid and high-quality service from the base of the breakdown, and England will look to capitalise on that when bidding to get back to form after six defeats in nine matches.
Jonny May has also come in from the cold to start on the wing, another to have missed out on the original squad selection. The Gloucester flier was recalled in the wake of the luckless Anthony Watson’s calf injury.
The 33-year-old has forced his way into the starting XV to take on the Pumas ahead of Max Malins and Henry Arundell.
Head coach Steve Borthwick has made two bold calls in promoting both Mitchell and May, selecting on form from the final warm-up matches and training in Le Touquet earlier this week.
England arrived in Marseille on Thursday afternoon, and will gear up to face Argentina in sweltering conditions in the ancient port city.
“With an excellent week of preparation in Le Touquet, the players are now looking forward to starting their Rugby World Cup campaign under the lights in Marseille on Saturday,” said Borthwick.
“After a number of impressive performances in the last 12 months by Argentina, we know that the first game in our pool will provide a stern test.
“This great tournament provides an excitingly special opportunity for our players, who I know, as ever, will be giving their all in front of our travelling support in the south of France.”
Leicester’s veteran prop Dan Cole will start at tighthead prop, the 36-year-old recalled by Borthwick in one of his first acts as head coach for the Six Nations in February.
Cole admitted earlier this week that he thought his Test career was over after the 2019 World Cup, where he was pressed into service for 77 minutes in the 32-12 final defeat by South Africa.
Kyle Sinckler was knocked out in the third minute of that loss in Yokohama, and Cole then had to battle two sets of near-peerless South Africa front-row forwards.
A valiant but creaking showing led to a three-year international absence, but now he is back in the fold – and selected ahead of both replacement Will Stuart and Sinckler, who has not made this weekend’s 23.
Tom Curry is another big selection at openside flanker, with the Sale star now fully fit after a frustrating ankle problem.
Curry played no part in England’s August warm-up matches after suffering his injury in training early last month.
The 25-year-old’s last competitive match was the Premiership final, where his Sale side lost to Saracens 35-25, on May 27.
Despite four months without a match, both Borthwick and defence coach Kevin Sinfield have insisted they have no qualms about throwing the destructive breakdown operator back in from the start.
Courtney Lawes will captain England on his 101st cap, with squad skipper Owen Farrell suspended. Ben Earl starts at No 8 with Billy Vunipola also suspended, while behind the scrum Manu Tuilagi and Joe Marchant form a complementary centre partnership.
England team to face Argentina in Rugby World Cup opener
Starting XV: F Steward; J May, J Marchant, M Tuilagi, E Daly; G Ford, A Mitchell; E Genge, J George, D Cole; M Itoje, O Chessum; C Lawes (capt), T Curry, B Earl
Replacements: T Dan, J Marler, W Stuart, G Martin, L Ludlam, D Care, M Smith, O Lawrence