The winger's promotion to the senior setup comes after a sparkling beginning to the new season for Dortmund that has included two Champions League starts.
Eyebrows were raised when he left Manchester City ahead of last term, but the risky switch already appears to be paying off with Southgate excited to see how Sancho adapts to international football.
"It's been a brave decision by him to play in the Bundesliga, but he is playing in front of big support every week. He's dealing with that and excelling," the England manager said.
"[Sancho and Mount] are both in their formative stages but we think they can play in the group.
"It's a big call, but it aligns to what we've done in the last 18 months. We have some injuries in midfield, so there's an opportunity there.
"We think there are some exciting young players we have been tracking, and it's a great opportunity for us to look at them.
"We are always conscious of the balance on how quickly we put those players in, but we think they are performing well with their clubs, they're confident players and we get an understanding of how they and we work."
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.
The Dortmund teenager also has managed five assists in the Bundesliga this season, which is good for the league lead despite only playing 125 minutes in six substitute appearances this season.
5 - Jadon Sancho has provided more assists than any other player in Europe's Big 5 Leagues this season (5), setting up a teammate once every 25 minutes on average in the Bundesliga. Starlet. pic.twitter.com/TUb8dWlD25— OptaJoe (@OptaJoe) October 4, 2018