Real wrapped up the title with a 3-0 win at Athletic Bilbao that maintained their seven-point lead over Barcelona with two games left and ended their bitter rivals' three-year stranglehold on the Spanish league championship.
Mourinho became the first coach to win a domestic league in four different major countries following his two titles in Portugal with Porto, two in England with Chelsea, two in Italy with Inter Milan and now one in Spain with Real.
"It was the most difficult," the 49-year-old said in a television interview.
"Up until the final moment nobody gifted us anything," he added.
"Bilbao put out their best team, the stadium was full and the team wanted to win and fight, which is the way it should be."
Getting the better of Barca, where he was an assistant under Bobby Robson in the late 1990s, will be a particular source of satisfaction for Mourinho, who has made a habit of needling his former employers during his two years in the Spanish capital.
He could not help taking a dig at the Catalans, suggesting one of the penalty's they had been awarded in their 4-1 home win over Malaga earlier on Wednesday was "soft".
"Barca won a lot of games and tried to push the league as far as possible," he added.
"I think that they themselves, champions and a club with a great tradition, know that we deserve this league title."
