England face a difficult challenge to get out of their World Cup group, which features Italy, Uruguay and Costa Rica.
Roy Hodgson's men qualified for the finals in Brazil without losing a game in qualification, although friendly defeats to Germany and Chile served as a reminder of the task facing the 1966 World Cup winners.
However, England's preparations for this year's tournament started with a 1-0 win over Denmark on Wednesday, and Mourinho sees no reason why Hodgson's side cannot win the title for a second time.
Speaking after being unveiled as Yahoo's global football ambassador for 2014, Mourinho said: "England, everybody says 'no, no, no, no, no.' I always say, 'why not?' Because I don't understand why not.
"When I see the players and the experience of the players and the clubs where they play, I don't understand why one day it cannot happen."
The Portuguese conceded England do underachieve at major competitions and may have a difficult time avoiding doing so again in Group D.
"I think so (that England underachieve) because (the Premier League) is the best championship in the world; it is the most competitive one," he added.
"They (the players) know what the big stage is; they know how to play under pressure; they know how to play for the big clubs; they play in the Champions League which is probably the most difficult football in the world, so I don't see why they don't (perform at the World Cup). I don't see why. So one day they have to do.
"I know it is (a tough group), but I don't think Uruguay and Italy will find it easy to play against England.
"I think it is the kind of group where everybody is waiting for the first match, and the first defeat will put immediately one team in a very difficult situation.
"One of them will be out and one of them will be considered like a big favourite, but the reality is that the group is difficult so one of them must be out."
