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.
England manager Gareth Southgate, 48, revealed the reasons behind his contract extension. "I'm always trying to find that balance, the messaging has to be correct, but it's good for their development to see how they train with us".
Sancho signed a new contract with Borussia Dortmund earlier this week and now he has earned his first call up to England's senior team.
It's a big call, but it aligns to what we've done in the last 18 months. We think there are some exciting young players that we've been tracking, either through their clubs or our junior teams ...
Southgate has made a strong impact since taking over from the sacked Sam Allardyce in 2016, evidenced by England's performances at Russian Federation 2018, when the Three Lions reached its first World Cup semi-final since 1990.
Sancho has netted once in the Bundesliga this season, while Chelsea-owned Mount has struck three times for Derby County in the Championship and Leicester City's Maddison's 15 chances created are the most among Englishmen in the Premier League.
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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. The selection of Sancho and Mount, as well as set-piece specialist Maddison, is Southgate's way of confronting that problem.
England lost to Spain 2-1 in its first match in the newly formed Nations League.
Watford midfielder Nathaniel Chalobah and goalkeepers Alex McCarthy and Marcus Bettinelli are the other uncapped players included, while Ross Barkley has been recalled at the expense of Chelsea team-mate Ruben Loftus-Cheek.
Injuries have led Southgate to make changes to World Cup semi-finalists England's squad.
Southgate, who became England coach in November 2016, is contracted to stay in charge of the national team through to the 2022 World Cup in Qatar.
"Experiencing at first-hand how the nation united behind the team this summer was something special", Southgate said, "and it will be great to see how far this young squad can go in the years to come".