The Briton took the lead off the line from Nerea Marti, who had a poor getaway, before she was passed by Kimilainen into Turn 11 with less than 10 minutes remaining.
A safety car ensued, before racing resumed on the last lap and the Briton reclaimed the lead into Turn 1, with the Finn running wide.
She held onto second place until a dramatic spin after contact with Marta Garcia at Turn 17 left her to fall down the order and finish last.
The incident allowed Garcia to snatch second, with Jessica Hawkins avoiding the Finn to take the final podium spot, having started sixth.
Marti's poor start allowed Chadwick into the lead, while Alice Powell stalled on the grid from third and Belen Garcia suffered a similar fate in seventh.
The safety car was deployed for the first time barely half a lap into the race after Powell's afternoon went from bad to worse when she hit the wall at the exit of Turn 7, which she blamed on dirt off the racing line.
A problem with the JCB removing her stricken car prompted a red flag with 18 minutes left on the clock and caused a 10-minute stoppage to the action.
Chadwick bunched up the pack for the restart, but Kimilainen was hot on her gearbox, while Fabienne Wohlwend and Garcia tussled for third, with the Spaniard emerging in front after Wohlwend went wide at Turn 11.
The safety car was deployed again with less than 10 minutes remaining after Wohlwend again went wide, making contact with Abbie Eaton at Turn 2 before the pair collided and ended their races at Turn 6, as Wohlwend's late braking putting her into the side of Eaton.
A last lap shootout ensued once the safety car period ended, with Chadwick snatching the lead after Kimilainen went wide at Turn 1, also allowing Garcia through to second place.
The pair made contact in the final seconds of the race, with Kimilainen forced into a spin and sending her to the back of the pack, while Garcia emerged unscathed to take second.
Beitske Visser finished in fifth, having started in 12th, with Abbi Pulling in sixth, up three places.
Chloe Chambers was the top rookie, finishing eighth, while Marti recovered to ninth after her poor start.
The second race of the season-opening double header takes place on Sunday at 15:35 BST, with Chadwick starting on pole.
