Mulder, who is the stand-in captain for his country's second Test against Zimbabwe, surprisingly declared at lunch with his team 626/5 as he stood just 33 runs from the highest score in Test Cricket history, which is held by Brian Lara following his 400 not out against England in 2004.
Mulder's 367 is the fifth highest score ever in Test cricket, behind Sri Lanka's Mahela Jayawardene (374), Lara again (375) and Australia's Matthew Hayden (380).
The 27-year-old broke several records as he smashed the ball to all parts. He became the first player to score a triple-hundred on his debut as captain, while having the highest individual score away from home.
On top of that, he became the second quickest player to ever score a triple hundred (297 balls), just behind Virender Sehwag's 278-ball effort against South Africa in 2008.
South Africa lead the two-match series against Zimbabwe 1-0 and are in a strong position to seal the whitewash.