At the close on day three in the series finale, Bethell had conceded England's big-name players were beginning to tire physically and mentally after a long tour.
He excluded himself from that group given he only came in for the fourth Test after previous No.3 Ollie Pope was axed.
But Wednesday's breakthrough century was the latest indication the 22-year-old Bethell will be one of the big guns by the next time England face Australia for the Ashes urn in 2027.
Barbadian-born Bethell was cautious, teetering on 99 runs for three overs, before raising his bat after lifting Beau Webster past deep mid-wicket for four around 4.15pm.
"A star is born here at the SCG," England legend Michael Vaughan said in commentary for Fox Cricket.
Bethell became only the 47th player to score a maiden first-class century in a Test match, joining superstars Kumar Sangakkara, Jason Gillespie and Jason Holder in the elite club.
He is also the first English specialist batter in history to do so.
Bethell was just the third man since the start of the 20th century to hit a maiden first-class century in an Ashes Test, behind Ian Healy and Jack Russell.
The young No.3 was called to the crease with England in dire straits, having lost opener Zak Crawley in the fifth ball of the first over while still 179 runs behind Australia's formidable first-innings total of 567.
By the time Bethell reached triple figures, the tourists were 30 runs ahead and had kept their slim hopes of victory alive.
England's most prolific Test batter Joe Root is the only other man to have scored a century for the visitors on a tour largely defined by the difference in quality between the sides' batting orders.
Bethell needed to navigate the loss of first-innings centurion Root (6) shortly after raising his bat for a fourth half-century from six Test matches so far.
He was made to work hard for his ton, notably wearing a Cameron Green bouncer on his helmet and falling to the ground just before lunch.
Team staff attended to Bethell on the field but he was able to play on in scorching hot Sydney conditions.
Bethell equalled his previous Test highscore of 96 with a four off Michael Neser past deep mid-wicket early in the final session.
He survived a big LBW appeal three balls later before charging down the wicket to put Webster's off-spin away before raising his bat.
The question now will be what might have been had left-hander Bethell been given a chance while the series was live.
Bethell needed to wait until Melbourne for his chance with Pope making only one score above 40 in the three Tests before that.
Bethell reached 40 in two of his four knocks.
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