Marsh was a surprise selection for Australia in the First Test of the hotly-anticipated series in South Africa against the No. 1 Test nation with the West Australian picked ahead of Phil Hughes and Moises Henriques, and just over a week after being ruled out with a calf injury.
However, he arrived in Centurion on Sunday and was picked to play on Wednesday, and has now delivered an unbeaten century at stumps on the opening day with Australia in a commanding position at 4-297.
Marsh was handed two lives in his knock and despite still being hampered by his calf at times with his running between the wicket, he finished the day not out on 122 from 232 balls with 12 boundaries.
Steve Smith is still with him as well as Australia recovered from 4-98 to 4-297 at stumps with Smith not out on 91 from 178 deliveries with his partnership with Marsh now unbroken on 199 for the fifth wicket.
Despite having only one Twenty20 innings back under his belt since injuring his calf and having nothing but good limited overs form to back up his selection, Marsh didn’t let the selectors down on his second, and potentially last, try at Test cricket.
The 30-year-old has only averaged 31 this Sheffield Shield season for Western Australia and has a mediocre career average of 35 while averaging just 27 in his seven previous Tests for Australia, but he showed against South Africa's much-vaunted pace attack that he has international-calibre talent.
Backing that up now will be the key and what has been the question over the left-hander, but on Wednesday at least he showed his ability by scoring is second Test century and making South Africa's much-vaunted bowling attack have to work extremely hard.
South African captain Graeme Smith won the toss and sent Australia in and would have liked that decision at 4-98, but now at 4-297 Australia is well and truly on top.
Dale Steyn had been sick overnight and wasn’t quite himself on Wednesday finishing the day with 2-54 from 20 overs with Morne Morkel and Jacques Kallis' replacement, Ryan McLaren the other wicket-takers.
Things started well for Australia with nine coming from Steyn's opening over, but the runs dried up shortly after that and the pressure told on David Warner.
The left-hander played a rash cut shot that he just managed to chop back onto his stumps off the bowling of Steyn to be out for 12 with the score 1-15.
Chris Rogers was struggling and had been given a good working over by Morkel, and one ball after being struck on the arm he was found out caught at short-leg by JP Duminy off the giant quick.
Rogers made just four with Australia 2-24 with debutant Alex Doolan and the recalled Marsh coming together with plenty resting on their shoulders as the only two players in the side who hadn’t played in the Ashes triumph.
Even though Marsh was dropped on 12 by Hashim Amla from the bowling of Morkel, the two were looking comfortable at the crease before Doolan attempted a pull shot off McLaren shortly before lunch.
He didn’t make good connection and hit it to Robin Peterson at mid-wicket who took a sharp catch to see Doolan's first Test innings come to an end on 27.
Australia was then 3-72 and shortly after lunch captain Michael Clarke's innings that he never looked comfortable in ended on 23 when Steyn claimed his scalp.
Australia's innings was then in danger of falling apart at 4-98, but nobody told Marsh and Smith that.
Smith continued on his brilliant form that he finished the Ashes with while Marsh was desperate to make the most of potentially his last chance at Test cricket even if he was hobbled and had no white cricket form to speak of backing him up.
After being dropped on 12, Marsh should have been run out on 84 with the score 4-201 but McLaren didn’t get back behind the stumps and nor did he gather in Steyn's throw from the outfield.
If he did either, Marsh would have still been well short of his ground but it wasn’t to be and he went on to score his century, and remain 122 not out at stumps with Smith with him on 91 and looking to record his fourth Test century on Thursday.