Mourinho signed Ballack on a free transfer from Bayern Munich for Chelsea in the summer of 2006 but the pair spent just one full season together at Stamford Bridge before the Portuguese was sacked.
Ballack won the FA Cup and League Cup during his time under Mourinho, who left a lasting impression on the former Germany captain.
Mourinho remains one of the best managers in Europe, having won league titles in Portugal, England, Spain and Italy, but he has often been criticized for his man-management, which has seen him fall out with great players like Iker Casillas, Ricardo Carvalho, Paul Pogba and Anthony Martial.
Nonetheless, Ballack says he always felt comfortable working with the two-time Champions League winner.
"You really get caught by his personality and by his ambitions," Ballack said to Sky Sports' Transfer Talk.
"Working with a coach like that was something totally new for me. He was impressive and I felt totally comfortable with the decision [to sign for Chelsea].
"The relationship is still good with him. You will not find many players who speak badly about him."
Of Mourinho's sudden Chelsea exit in September 2007, Ballack added: "I was experienced enough to understand.
"When you are not winning, the coach is more or less the weakest person in the team who can be replaced.
"He was in his fourth year. If you look at the average time a manager has to work in a club, it was quite a good time.
"The way he worked was so intense that maybe you come to a point where things are not working anymore. Even then, the fans know what he achieved at the club to take them back to that level."
