French fans went wild after Leon Marchand won gold in the 200m butterfly. (0:37)
Spaniard Maria Perez was second in 1:26:19 and the fast-finishing Montag broke her own national record with 1:26:25 for bronze.
Australia's Jemima Montag during the Women's 20km Race Walk David Davies/PA Images via Getty Images
The only other Australian women's walker to secure a podium finish at Olympic level was Jane Saville, who was third in the same event at the 2004 Athens Games.
A noted big-event performer, the 26-year-old Montag was second at last year's world championships and claimed gold at the previous two Commonwealth Games.
Earlier on Thursday, Brian Pintado from Ecuador won the first athletics title of the Paris Olympics, surging away in the final stages to claim gold in the men's 20km walk.
Pintado clocked a winning time of 1:18:55, with the minor medals going to Caio Bonfim from Brazil (1:19:09) and Spaniard Alvaro Martin (1:19:11).
