Parkes, 30, was named at inside centre on the very day he qualified for Wales after serving three years of residency and the Kiwi farmboy produced a strong-running and skilful display as Wales looked like they might squander an 11-point half-time lead.
"It's been amazing. A dream come true," man-of-the-match Parkes told BBC.
"To be able to get a meat pie (try) and score a double on your debut is very special so I'm very happy."
The victory meant Wales finished the autumn series with two wins, their scrappy triumph over Georgia sandwiched between defeats by Australia and New Zealand.
"We wanted to finish 2017 off for the fans," said Wales skipper Alun Wyn Jones. "We had two good halves against Australia and New Zealand and we had a purple patch from the first whistle but we managed to finish on a high.
"We're disappointed with some of the results, but we're looking to change things and come back stronger next year."
But question marks will surely be raised over the future of Allister Coetzee, who has led South Africa to wins over France and Italy over the last two weekends, but whose side have gone down to record defeats by Ireland and the All Blacks.
"We're very disappointed," admitted 'Bok flanker Siya Kolisi.
"The way we started today wasn't good enough and we didn't have the patience. We have to be more disciplined at the end of the game and not make silly mistakes. We have to be better than this next year."
The match at the Principality Stadium came to life after five minutes, Northampton-bound fly-half Dan Biggar twice pulling the strings to great effect.
Firstly winger Hallam Amos latched on to an inch-perfect crosskick from the fly-half and cut back inside Springbok full-back Andries Coetzee before offloading to Scott Williams, who had a simple run in.
No sooner had the restart been gathered than Biggar put in a deft chip behind a flat 'Bok defence which Parkes scooped up to slide over for a second five-pointer.
Halfpenny converted the two tries and Welsh tails were up although the South African scrum looked by far the more dominant.
No cigar for Marx
South Africa's fly-half Handre Pollard spurned a shot at the posts after Wales strayed offside, and a devastating driving maul off clean line-out ball saw Malcolm Marx crash over in the corner, only for French referee Jerome Garces to harshly call an unclear grounding.
Pollard eventually got the visitors on the scoreboard on the half-hour mark with a penalty after Josh Navidi went high in the tackle. But any threat of an immediatate comeback was shortlived as Biggar charged down a clearance kick, Taulupe Faletau seized the ball and flung a pass back inside to Parkes who went in for his second try.
Another intercept and Biggar put in a kick chase, but Dillyn Leyds was given all the time in the world, the winger setting off on a mazy run before finding Jesse Kriel, the centre putting in a perfectly-weighted grubber that debutant Warrick Gelant dotted down ahead of the covering Aled Davies.
Pollard converted to make it 21-10 at half-time and the 'Boks opened the second period seemingly focused on less chaotic rugby and with a gameplan based around their powerful forwards led magnificently by go-to hit-up man Steven Kitshoff.
The pressure told, Pollard crashing over after several phases of drives close to the Welsh line.
With Wales tight-head Scott Andrews, in his first Test since 2013, targeted in the scrum, the 'Boks played it tight, eventually freeing Kriel, who powered through Halfpenny for a try Pollard converted from the touchline to hand the Springboks a 22-21 lead.
But Halfpenny booted a penalty for the home side to regain the lead with 12 minutes to play. A raft of replacements broke the tempo of the game and Wales desperately held out for a much-needed victory which had looked a lot more academic at half-time.