Mantashe cautioned that dismissing allegations against ANC cadres was doing vast damage to the party's image, the Sunday Independent newspaper reported.
"The many high profile cases and allegations against our comrades are not helpful. We can be dismissive of them but the damage on the ANC image is huge," it quoted him as saying.
Mantashe reportedly made the comment in a nine-page report drafted for the ANC's lekgotla, which concludes on Sunday.
"One issue that constantly cropped up in the elections research, even among our staunchest supporters, is that the ANC is soft on corruption and looks after their own," Mantashe is further quoted as saying.
"This requires a system for processing such allegations that will send a message of an ANC that is intolerant of corruption."
The warning comes as President Jacob Zuma faces pressure to act against Public Works Minister Gwen Mahlangu-Nkabinde and national police commissioner Bheki Cele.
Public Protector Thuli Madonsela found this week that they were responsible for maladministration regarding the lease contracts for new police headquarters in Pretoria and Durban.
Cosatu and opposition parties have said Cele and the minister must resign.
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