Totally overwhelmed in the first set, Mladenovic (29) looked like she could turn the game around when she in turn went 3-0 in the second set with a double break.
Fernandez recovered part of his delay in stride to come back to 3-2, but Mladenovic, ex-N.10 in the WTA, broke away 5-3 and had two set points on his serve at 5- 4.
But it was ultimately the 19-year-old Canadian who lined up four games in a row to win the match by taking advantage of a double fault from Mladenovic on match point.
She will face the Czech Katerina Siniakova (56th) in the next round to try to climb to the 3rd round of the Major on clay, as in 2020.
Mladenovic has won just two of the 13 matches she has played this year, including one in Madrid qualifying where she failed to reach the main draw.
Then, she eliminated her compatriot Tessah Andrianjafitrimo (142nd) in the first round in Rabat just before coming to Roland-Garros.
She had reached the quarter-finals at Porte d'Auteuil in 2017, two years after having done so at the US Open.
Of the six French people competing on Sunday, only Grégoire Barrère made it to the second round.
Like Mladenovic, Harmony Tan, Carole Monnet and Clara Burel were eliminated in the women's draw, while Quentin Halys lost in the men's.
© 2022 AFP