On the same day Root defended Jamie Smith over his ugly dismissal in the Ashes finale, the No.4 helped the tourists to their largest innings total of the series.
With 160 from 242 balls, Root powered England to 384 in their first dig and was only dismissed by a freakish caught-and-bowled effort from Michael Neser (4-60) late in day two's second session.
Root shrugged to the crowd in celebrating his heroic century, after playing Neser down the ground for two runs to raise triple figures.
He'd performed the same celebration when he broke through for a long-awaited first ton in Australia at the Gabba last month.
"I just felt like we've had some amazing support throughout this series and we've not been able to achieve what we set out to as a group, but at no point has that (support) ever wavered," he said.
"It (the celebration) is a way of saying thank you, really."
Only a matter of weeks ago, Root had carried the weight of expectation on those same shoulders, winless in his first 17 Tests in Australia and failing to score a century in his first 29 innings here.
He will leave Australia with two tons and a Test match win at the MCG, though not the Ashes urn as hoped.
No other man has yet scored a century for the visitors this summer, with Root's latest triple-figure score lifting him to equal-third on the all-time standings alongside Ricky Ponting, with 41 tons apiece.
It's conceivable Root could score 1,985 more runs and pass Sachin Tendulkar to become the greatest run-scorer in Test history should he play on until England's next Ashes tour.
Second on the standings, the former England captain is unsure if he'll have retired by that 2029/30 summer, during which he will turn 39.
"Who knows? We'll see. I'd love to (play until then), but we'll see how things unfold in time," Root said.
Root's teammates gave too little assistance as England fought to put some respectability on a series scoreline currently sitting at 3-1.
Without gun pacers Jofra Archer and Mark Wood, England had no answers for player-of-the-series contender Travis Head (91no) once he began rolling through his arsenal of shots.
Streaky opener Jake Weatherald (21) was twice dropped - one a regulation chance from Ben Duckett at mid-wicket - and frustrations were clearly mounting by the last half hour of play.
Ben Stokes exchanged words with Marnus Labuschagne (48) after Head hit him for two consecutive fours to end an over.
The English captain dismissed the Australian No.3 caught-behind on the next delivery he bowled to him.
Perhaps the most egregious moment came as Jamie Smith (46), closing in on a half-century, played a Labuschagne short ball straight to Scott Boland at cover just before lunch.
It was the first Test wicket Labuschagne had taken with medium pace, but Root did not feel Smith's dismissal was as mindless as it may have looked.
"It was about almost maximising that 10-over period ahead of facing that new ball," he said.
"You eke out an extra 20 runs, that could be the difference later on down the line. There was method behind what we were trying to do. We're trying to move the game forward all the time.
"When it doesn't come off, it can look a certain way. But you're never playing to get out, you're playing to score runs."
Root stumbled into the pavilion with back soreness around 4:15pm and did not return but is expected to return to the field on Tuesday.
"It was a bit embarrassing; it just cramped," he said.
"But calmed things down and thankfully now, with a lot more fluids on board, I feel fine."
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