And Southgate has included Mount, the Chelsea attacking midfielder now on loan at Derby County, in his selection for the first time, while Leicester City star Maddison and Borussia Dortmund's Sancho are also in line to win their first caps.
The 18-year-old Sancho was called up having already played in the Champions League for German club Dortmund, with Mount rewarded for some strong displays at English second-tier club Derby after joining on loan from Premier League giants Chelsea.
Reports suggested he would be handed his first U21 call but an impressive performance and an assist against Monaco on Wednesday changed Southgate's mind.
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"We'd have no hesitation in playing them in the games", Southgate said of Mount and Sancho.
Chelsea midfielder Ross Barkley returns to the squad for the first time since May 2016.
The likes of Ross Barkley and Danny Welbeck are also included after performing well for Chelsea and Arsenal, but Ruben Loftus-Cheek misses out, as does Burnley number one Joe Hart.
Dele Alli, Fabian Delph, Jesse Lingard and Adam Lallana all miss the two Nations League fixtures through injury.
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"He's a player that is explosive, is really creative in the final third, and has had a big impact in terms of assists in the matches he has played, can score goals and has had a big impact in our junior teams in the last couple of years".
"You never know with young players what they are capable of until they have an opportunity", the manager told a news conference at the national St George's Park training center.
England manager Gareth Southgate, 48, revealed the reasons behind his contract extension. Mount has created more chances than any other player in the League Championship and scored for England's under-21 team last month.
Southgate joked that assistant Steve Holland, who has also signed on until 2022, had been watching Sancho disguised in "a flat cap and glasses" in Germany, where the teenager's displays were enough for a call-up regardless of the current injury issues.
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"Mason's is a slightly different situation". The squad has a total of 51 worldwide goals.