Mr Mani, a state Secretariat member of CPI(M) and first-time MLA, represents Udumbanchola constituency in the hilly Idukki district. He is likely to be sworn in on November 22.
He has also served as the party's Idukki district secretary for many years.
Party state secretary Kodiyeri Balakrishnan made the announcement after a three-day state committee meeting, which ended on Sunday.
Kodiyeri told reporters that the party had recommended induction of Mani as there was a vacancy following resignation of Industries Minister E P Jayarajan, who was the number two in the cabinet.
Mr Jayarajan was forced to quit on October 14 following allegations of nepotism in key appointments in state public sector undertakings.
Mr Mani had earlier stirred a controversy by his statement that CPI(M) had often eliminated its political foes in Idukki district in the 1980s, following which police had registered a case against him for murder and other charges.
He was arrested in November 2012 in connection with the case registered for murder of Anchery Baby, a Youth Congress leader, who was shot dead on November 13, 1982.
A visibly elated Mr Mani told reporters that he was very happy and "indebted" to the party for deciding to make him a minister.
"My becoming minister was totally unexpected," he said.
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