The Briton, hoping to take her seventh consecutive series win in France, faced a stern threat to her reign from the young Spaniard, who held provisional pole for much of the session.
But Garcia was unable to better Chadwick’s time, finishing in fourth, with a last-ditch attempt from Beitske Visser putting her up into second, just 0.011 seconds behind the leader, while Nerea Marti improved to third.
Chadwick, driving for Jenner Racing, faces an investigation after the session for crossing the line at the pitlane exit.
Fabienne Wohlwend led the field out as the sun set on Le Castellet, closely followed by Chadwick, who has won every race so far this season.
The benchmark was set at 2m04.825s by the Briton, with Garcia quick to better it by 0.2s.
Chadwick improved by 0.5s on her second attempt, but Garcia once again went top, with Belen Garcia slotting into second with a 2m03.806s.
Marti was next to challenge for provisional pole, with a 2m03.596s putting her into contention behind Chadwick.
The Briton clung onto third with her next attempt, a 2m03.665s, before Visser took second with a 2m03.473s.
The Dutch driver improved on her next run, knocking Marta Garcia off the top spot with a 2m03.268s.
The field pitted for fresh Hankooks at the halfway mark before Marti put in the quickest time, having got her tyres in the window quickly to set a 2m03.150s.
Marta Garcia was next to reclaim the top spot, almost 0.1s quicker than Marti while, further back, Alice Powell, who was struggling for pace having not driven at the French circuit before, slotted into eighth.
As the times tumbled, provisional pole changed hands several times, with Visser taking the top spot first before Marta Garcia went quicker, with Chadwick settling for second, 0.045s behind the leader.
Heading into the final five minutes, Marti once again took the advantage, with a 2m02.339s, while Chadwick was unable to improve on fourth place.
But in the final two minutes, Chadwick managed a 2m02.235s to snatch pole, just 0.1s clear of Visser, who also improved.
Belen Garcia took fifth, with Abbie Eaton in sixth and Wohlwend in seventh. Jess Hawkins was eighth, with fellow Briton Sarah Moore in ninth and Emma Kimilainen rounding off the top 10.
Starting grid:
| Cla | Drivers | Time | Gap |
|---|---|---|---|
| 1 |
 Jamie Chadwick | 2'02.235 | |
| 2 |
 Beitske Visser | 2'02.246 | 0.011 |
| 3 | Nerea Martí | 2'02.339 | 0.104 |
| 4 |
 Marta Garcia | 2'02.359 | 0.124 |
| 5 | Belén García | 2'02.572 | 0.337 |
| 6 |
Abbie Eaton | 2'02.918 | 0.683 |
| 7 |
 Fabienne Wohlwend | 2'02.922 | 0.687 |
| 8 |
Jessica Hawkins | 2'02.986 | 0.751 |
| 9 |
Sarah Moore | 2'03.009 | 0.774 |
| 10 |
 Emma Kimilainen | 2'03.018 | 0.783 |
| 11 |
Alice Powell | 2'03.139 | 0.904 |
| 12 | Abbi Pulling | 2'03.345 | 1.110 |
| 13 |
 Bruna Tomaselli | 2'03.475 | 1.240 |
| 14 |
 Juju Noda | 2'03.671 | 1.436 |
| 15 | Tereza Bábíčková | 2'03.828 | 1.593 |
| 16 | Emely de | 2'04.016 | 1.781 |
| 17 | Chloe Chambers | 2'04.027 | 1.792 |
| 18 | Bianca Bustamante | 2'05.659 | 3.424 |
