The defender's form has been a media talking point during recent weeks, following some disappointing United results.
But the Three Lions boss said he never even considered omitting Maguire, due to the centre-back's huge contribution to England's progress over the last four or five years.
At the press conference where the 25-man squad was announced, Southgate responded to several questions about the United players eligible for selection.
“I don’t think it ever crossed our minds not to select Harry Maguire for this squad,” explained the former England defender.
“He’s been a key part of the starting XI that got us to a World Cup semi-final and a Euros final.