The club will officially change hands on 15 June at the general assembly of Olimpija Ljubljana, when Rastoder, who confirmed the sale for the STA, will bid farewell from the club that he has also chaired for five years.
The daily Delo reports on Tuesday that the transaction is worth EUR 4m.
The owner of banana wholesaler Rastoder is leaving the premier league club from the capital without winning any major titles. The club finished fourth in the national championship this season, 18 points behind the champions Maribor.
According to Delo, Rastoder has fallen out of favour of the fans due to the lack of success. The fans have also boycotted Olimpija's home games to show their disagreement with the club's policies.
Taking over is the 77-year-old Mandarić, a Croatian-born Serb who has owned a string of successful businesses and football clubs including England's Portsmouth, Leicester City and Sheffield Wednesday.
Delo says that Mandarić's right-hand in Olimpija will be Ranko Ostojić, a former goalkeeper, who is to be named director general and who is already looking for coaches for the new season, which starts on 18 July.
According to the paper, the most serious candidates are former Slovenian national team head coach Slaviša Stojanović and former Partizan Belgrade coach Marko Nikolić.