The winning bidder stumped up USD 655,000, around double the projected selling price, for the property in the upmarket Sandown area of the city.
The mansion has four bedrooms and bathrooms, a cinema room and cigar lounge, a strong room and staff living quarters.
Malema, the former youth leader of the ruling African National Congress, reportedly bought the property in 2009 for 3.6 million rand, demolished the existing house and starting construction anew.
The buyer was named as Norman Tloubatla, chief executive of local firm Magnified Designs.
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Once a vocal supporter of President Jacob Zuma, Malema was kicked out of the ruling party last year for insubordination.
His fortunes have since taken a turn for the worse, leading to suggestions of political persecution.
The South African Revenue Service rejected his offer to pay a quarter of his 16-million-rand tax bill from 2005 to 2011, blasting his poverty plea as a lie.
They seized his two houses, a farm and a smallholding to recover the arrears.