Mitchell and Walker-Peters will also be joined in the squad by West Brom goalkeeper Sam Johnstone and Aston Villa forward Ollie Watkins after the withdrawals of Aaron Ramsdale, Trent Alexander-Arnold, Reece James and Tammy Abraham through injury.
Southgate met with the rest of the England coaching staff on Monday to assess those who might withdraw, and also decide on which players to add to his squad.
He had a major problem in defence, with Luke Shaw the only fit full-back remaining on his list - and he has not played since the end of February, after recovering from injury and a bout of Covid.
Liverpool's Alexander-Arnold is also out for a few weeks with a hamstring strain, and Chelsea head coach Thomas Tuchel has made it clear he wants James to stay with the Blues to recover from injury, rather than join up for two friendly internationals at Wembley.
It means call-ups for Walker-Peters, who is a 2017 FIFA U20 World Cup-winner and has amassed caps from U18 through to U21 level for England, and Mitchell, despite the Palace full-back having never played in any of England's junior teams.
The 22-year-old has played 33 matches for Patrick Vieira's side this season and has caught the eye of the England boss.
"Tyrick, we've been really impressed with, he was very close [to a call-up]," Southgate said last week. "I think he's been very good with his defending, he gets out quickly to people and in the biggest games against top opponents he's coped really well."
England also had problems with their goalkeepers too, with Arsenal goalkeeper Ramsdale ruled out for a few weeks with a muscle strain, and Jordan Pickford still recovering from illness - though he did play in Everton's 4-0 defeat to Palace yesterday.
Johnstone, who has three senior caps to his name, has been called into the squad as back-up for Pickford and Burnley's Nick Pope, while Abraham's withdrawal means another opportunity for Watkins, who scored on his England debut against San Marino in March 2021.
Southgate has given England's players an extra day off, as they head into their first international training camp of 2022, with the squad due to report at St George's Park on Tuesday.
England host Switzerland on Saturday March 26 (kick-off 5.30pm) at Wembley Stadium before facing Ivory Coast on Tuesday March 29 (kick-off 7.45pm), also at Wembley. Both games are live on Sky Sports.
