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.
ENGLAND SQUAD TO PLAY CROATIA AND SPAIN: J Pickford (Everton), M Bettinelli (Fulham), J Butland (Stoke), A McCarthy (Southampton); T Alexander-Arnold (Liverpool), J Gomez (Liverpool), H Maguire (Leicester), D Rose (Tottenham), L Shaw (Man Utd), J Stones (Man City), J Tarkowski (Burnley), K Trippier (Tottenham), K Walker (Man City); N Chalobah (Watford), R Barkley (Chelsea), E Dier (Tottenham), J Henderson (Liverpool), J Maddison (Leicester), M Mount (Derby), J Sancho (B Dortmund), H Winks (Tottenham); H Kane (Tottenham), M Rashford (Man Utd), R Sterling (Man City), D Welbeck (Arsenal).
The 19-year-old Mount, a member of Chelsea's FA Youth Cup-winning teams of 2016 and '17, spent last season on loan at Dutch club Vitesse Arnhem and is excelling for a Derby side that is bidding for promotion back to the Premier League under Frank Lampard, a former Chelsea player.
Tottenham midfielder Harry Winks, whose only cap came against Lithuania in October 2017, returns to the squad. These calls however are likely to be resisted by Southgate for now, in favour of letting Sancho improve with Aidy Boothroyd's Under-21 side first.
Dele Alli has been sidelined by a hamstring problem, while fitness doubts have seen the likes of Fabian Delph, Adam Lallana, and Jesse Lingard omitted.
I have learned a great deal from them and their passion and professionalism in representing England has been an inspiration to my own work - in particular Steve Holland, who I'm pleased to be continuing to work alongside through to 2022. "He's been fearless enough to go overseas and play", added Southgate.
Mason's situation is slightly different.
"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".
"To have a player starting in the Champions League is of importance to us", Southgate said.
Only nine have scored for England.