Sources had told ESPN the 22-year-old was set to miss out but Southgate opted to exclude Southampton midfielder James Ward-Prowse instead given Alexander-Arnold's potential to play in midfield in addition to his regular role at right-back.
Maguire missed Manchester United's Europa League final defeat to Villarreal with an ankle ligament injury while Henderson has not played since February after undergoing groin surgery but both are viewed as key figures in Southgate's setup.
Southgate revealed that both players are doubts for England's Euro 2020 opener against Croatia on June 11.
Asked specifically about Maguire's availability for the first Group D game, he said: "We don't know that yet but it is going to be tight for the first game, there's no question. But he's such a good player and because we had the additional size of squad, the fact that leadership is growing in him all the time, we wanted to take the opportunity to bring him into the squad.
"We know exactly where he's at. Our medical team have been over with him in Manchester. He's not travelled yet to us because with the quarantine issues coming back from Poland, that was a bit more complicated. It's like Hendo, they've both got a bit to do to be available.
"But we feel that the possibility they can play a part and they are such fantastic characters to have around the group, Hendo's leadership, his presence is already having an effect on other people, especially the younger ones in the group but all of the others as well. Given we were able to pick 26, we felt the opportunity to have those two with us was a straightforward one really."
Leeds midfielder Kalvin Phillips was also an injury doubt with a shoulder problem but he was pictured returning to training in recent days and is included as is Arsenal winger Bukayo Saka, who has just four caps to his name.
Southgate had originally named an expanded 33-man squad amid various injury concerns and the presence of United, Chelsea and Manchester City in the European club finals.
The seven players omitted are Ward-Prowse, Sheffield United's Aaron Ramsdale, Manchester United's Jesse Lingard, Everton's Ben Godfrey, Brighton & Hove Albion's Ben White and Aston Villa's Ollie Watkins while United striker Mason Greenwood withdrew due to injury.
Alexander-Arnold has been given No. 22 with Saka wearing the No. 25. England's first warm-up game will take place in Middlesbrough against Austria on Wednesday evening featuring a much-changed line-up.
Ward-Prowse, Lingard and Watkins were all given the chance to go home but have opted to remain with the group and could play some part in the game with the players involved in the Champions League final between Chelsea and City not likely to join up with the squad until Friday.
Defenders: Trent Alexander-Arnold (Liverpool), Ben Chilwell (Chelsea), Conor Coady (Wolverhampton Wanderers), Reece James (Chelsea), Harry Maguire (Manchester United), Tyrone Mings (Aston Villa), Luke Shaw (Manchester United), John Stones (Manchester City), Kieran Trippier (Atletico Madrid), Kyle Walker (Manchester City).
Forwards: Dominic Calvert-Lewin (Everton), Phil Foden (Manchester City), Jack Grealish (Aston Villa), Harry Kane (Tottenham Hotspur), Marcus Rashford (Manchester United), Bukayo Saka (Arsenal), Jadon Sancho (Borussia Dortmund), Raheem Sterling (Manchester City).