After Australia slumped to 6-124 midway through the opening day of the series, Renshaw made the most of his surprise move from opener to No.5 to be 190no at tea on day two in Durban.
The runs came with superb support from Pat Cummins (43) and Nathan Lyon (27), with whom he added 84 and 134 runs for the seventh and ninth wickets respectively to completely turn the match.
Renshaw was a baby-faced 20-year-old when he scored 184 in his fourth Test against Pakistan in January 2017, before finding himself dropped 10 months later.
Some 3567 days on from that hundred, Renshaw is now a 30-year-old who looks a far more complete cricketer after earning his third recall as an opener in August..
So good was Renshaw's innings, Australia may now have another selection headache after Nic Maddinson came into the side and opened in this Test with a 33-ball duck.
If Renshaw is this settled in the middle order, can they afford to move the Queenslander back to open once Cameron Green or Josh Inglis comes back in?
For now, though, Australia will be relieved to have him at No.5 in this Test.
He was busy against the spin on Friday, and hit most of his early boundaries against the quicks behind square in the first half of his innings.
After bringing up his half-century in the first over on Saturday, Renshaw got through a tough period with the second new ball just after Cummins fell.
But as the morning went on Renshaw became more comfortable, cover-driving and cutting Anrich Nortje for two boundaries in one over.
He then hit his 204th ball from Marco Jansen to the long-on boundary to bring up his century, prompting a long embrace with Lyon and meaningful celebration.
After lunch he started reverse-sweeping the spinners to the boundary and even uppercut Nortje for a six, as his innings turned into the longest by an Australian batting at No.5 in 14 years.
South Africa were also guilty of letting the game slip away, failing to stop Renshaw from masterfully controlling the strike with Lyon as he faced the first ball of 13 straight overs.
Even still Lyon looked most comfortable when forced to navigate through his 92 deliveries and 192 minutes at the crease.
Together, the pair's partnership marked Australia's biggest for the ninth wicket since 1894.
The other concern for the Proteas was the lack of purchase they got out of their spin, after going with two tweakers.
While Simon Harmer (0-68) and Keshav Maharaj (0-75) are there for when the pitch dries later in the game, the fact they went wicketless was notable.
That allowed Australia to put plenty of overs into South Africa's quicks, being prepared to get into the grind after admitting that was an area that needed work.
Nortje (2-88) was forced to bowl 28 overs in his first first-class game in three-and-a-half years, while Wiaan Mudler (4-68) bowled 27 and Jansen 31 for his 3-68.
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