The 32-year-old lost his place in the side under Rafael Benitez last season and has had to deal with various off-the-pitch problems.
His manager insists that he is now over that, and is pleased that Terry is proving himself to still be an important player, having been an ever-present in the league this season.
"[Terry] is recovering his self-esteem," Mourinho told reporters.
"In the last few years he was not playing a lot, he had problems on the pitch, outside the pitch, suspensions, injuries, managers who did not trust him enough. It looked like his career was going in the wrong direction.
"Even I was questioning what was happening [but] I'm happy he's proving he's still a top player. He plays in a position where age doesn't make a huge difference."
Mourinho was speaking ahead of Chelsea's game against Cardiff City this weekend.