Gardner came to the crease at 4-59 in the 15th over at Hobart's Ninja Stadium on Friday and salvaged the innings alongside the experienced Beth Mooney (50).
The pair combined for a 95-run partnership before Mooney skied a Charlie Dean delivery with almost 18 overs left.
But Gardner kicked on for a run-a-ball 102 - her first triple digit score across all formats for Australia - before falling in the 46th over, caught in the deep off Lauren Bell.
MAIDEN TON!!— cricket.com.au (@cricketcomau) Ash Gardner brings up three figures for Australia for the first time, what a knock! #Ashes #AUSvENG pic.twitter.com/fC3zeyQjy6January 17, 2025
The in-form 27-year-old, who hit an unbeaten 42 to get Australia home in the opening ODI, surpassed her previous ODI best of 74.
Tahlia McGrath came to the party late, hitting 55 from 45 balls on the back of two low scores.
Legspinner Georgia Wareham, brought in to the side at the expense of quick Darcie Brown, struck an unbeaten 38 from 12 balls.
Australia holds a four-points-to-none lead in the multi-format Ashes series after winning the first two ODIs, both worth two points.
The three ODI games are followed by three T20s (two points each) and one Test (four points).
Earlier, after winning the toss, Australia's much-vaunted top three of Phoebe Litchfield (15), Alyssa Healy (15) and Ellyse Perry (two) failed to fire.
Litchfield was given not out on the field but an England review revealed she had gloved a Lauren Filer delivery to wicketkeeper Amy Jones.
Australia took the long handle to England's top-class spinner Sophie Ecclestone, who finished with 1-76 after taking six wickets across the first two ODIs.
Australia are chasing a sixth-straight Ashes series triumph and need eight points to retain the trophy.
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