Mladenov replaces former European Footballer of the Year Hristo Stoichkov, who resigned on July 8 after only a month in charge, saying he had lost faith in the club's then owners, Titan Sport, and their "endless lies".
Troubled CSKA, the 31-times Bulgarian champions, were subsequently taken over by a business group called "Red Champions" and Mladenov was rehired by the club just six months after he had been sacked following his third stint in charge.
"We are at the bottom and we have to start restoring our positions," Mladenov, who led CSKA to their last league title five years ago, told reporters.
The Reds hit difficult times of late with inept performances on the pitch combined with controversy in the boardroom and crippling debts.
The off-field problems have led to insecurity in the dressing room, with more than half a dozen key players, including captain Ivan Bandalovski and Brazilian striker Michel Platini, leaving the club in the last few weeks.
"We lost our image, our dignity not only in Bulgarian football but in Europe too," added the 56-year-old, who is known as "The executioner of Liverpool" in the Balkan country for his double in a 2-0 win over the English side in the 1982 European Cup which secured a semi-final place.
"But we'll instil a winning mentality in every player and at every match will chase the win. CSKA must start winning again but it takes time, this cannot happen for six months."
In his first spell, Mladenov won the league title in 2003. His third spell in the job began in March 2012 but he was unable to make any improvement and was sacked 10 months later.
CSKA, who reached three European semis between 1967 and 1989, finished third in the league last season, nine points behind champions Ludogorets.
Mladenov will make his official debut on Saturday when CSKA host Lokomotiv Plovdiv in the league's opening round of play.