The 21-year-old struggled for form under Mourinho last season but has begun the new campaign in fine style, scoring four goals and setting up two more as he alternates with Marcus Rashford as strike partner for new arrival Romelu Lukaku.
"I see a great improvement in the person, in the moods, in the face and body language," Mourinho told reporters today.
"If we want a word - happiness. He is a happy guy. He is working extremely well, he starts matches and tries to do well.
"He goes from the bench, even if for ten minutes like in the last match at home, he tries to enjoy and to give something in the extra minutes. I am very pleased with his attitude overall, and then it is easier to play well."
Martial is already halfway towards reaching his tally of eight goals in all competitions last season.