Mitchell was left out of Borthwick’s training squad at the end of June, but was called back into the 33-man group after an injury to Jack van Poortvliet.
The Northampton Saints half back has leapfrogged veteran nines Ben Youngs and Danny Care in the pecking order to start England’s first Pool D fixture at the Stade Velodrome on Saturday.
Jonny May is also a surprise starter, with the experienced wing similarly called into the 33 after Anthony Watson’s injury.
Tom Curry, who missed all four of England’s warm-ups while battling his own fitness issue, is able to return in the back row alongside Ben Earl and Courtney Lawes, who captains the side with Owen Farrell suspended.
“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.”
Up front, Dan Cole gets the nod at tighthead, with the prop uniting with certain starters Ellis Genge and Jamie George in the front row. Ollie Chessum and Maro Itoje combine at lock.
In Farrell’s stead, George Ford continues as the starting fly half, with Manu Tuilagi and Joe Marchant his midfield company.
Elliot Daly and Freddie Steward join May in the back three, while Marcus Smith could be deployed at full back or fly half from the bench. A fit-again George Martin is included as lock cover among the replacements.
England also face Japan, Chile and Samoa in Pool D.
England team to face Argentina in Marseille (kick off 8pm BST, Saturday 9 September)
15. Freddie Steward (Leicester Tigers, 26 caps)
14. Jonny May (Gloucester Rugby, 73 caps)
13. Joe Marchant (Stade Francais, 19 caps)
12. Manu Tuilagi (Sale Sharks, 53 caps)
11. Elliot Daly (Saracens, 59 caps)
10. George Ford – vice-captain (Sale Sharks, 85 caps)
9. Alex Mitchell (Northampton Saints, 6 caps)
1. Ellis Genge – vice-captain (Bristol Bears, 52 caps)
2. Jamie George (Saracens, 79 caps)
3. Dan Cole (Leicester Tigers, 102 caps)
4. Maro Itoje (Saracens, 70 caps)
5. Ollie Chessum (Leicester Tigers, 11 caps)
6. Courtney Lawes – captain (Northampton Saints, 100 caps)
7. Tom Curry (Sale Sharks, 45 caps)
8. Ben Earl (Saracens, 18 caps)
16. Theo Dan (Saracens, 3 caps)
17. Joe Marler (Harlequins, 82 caps)
18. Will Stuart (Bath Rugby, 29 caps)
19. George Martin (Leicester Tigers, 3 caps)
20. Lewis Ludlam (Northampton Saints, 21 caps)
21. Danny Care (Harlequins, 90 caps)
22. Marcus Smith (Harlequins, 24 caps)
23. Ollie Lawrence (Bath Rugby, 14 caps)