At 31, the Chilean-born skier who trains in France outpaced the leading favourites to clinch her first World Cup victory, 13 years after her only podium to date in a giant slalom in Sestriere.
Ruiz Castillo won in one minute 42.56 seconds to bridge a seven-year gap since the last top spot earned by compatriot Maria Jose Rienda Contreras in a giant slalom in Hafjell seven years ago.
It was almost a home win for popular speed specialist Ruiz Castillo as she regularly trains with the French team and had the chance to practise on the Meribel course, which had not been used on the World Cup circuit for 19 years.
"It's a formidable achievement after all the commitment and hard work for so many years in difficult conditions," she said.
"Fortunately, this year I had the opportunity to train well with the French and I knew I had the potential as I'm really strong in practice.
"It's a great day for me as the conditions suited me ideally," the Spaniard added referring to the smooth technical course.
Germany's Maria Hoefl-Riesch, the super-combined world champion in Schladming, was second, 0.20 seconds adrift for her third downhill podium of the winter after Lake Louise and her bronze medal at the worlds earlier this month.
"I cannot be disappointed to come second behind Carolina. Everybody's happy for her. It has not been a great season for me so second is a good result.
"Since I don't have a crystal globe to fight for, I'm going to go for race wins without pressure, hoping to do well next week at home in Garmisch," she said.
Local favourite Marie Marchand-Arvier missed the runners-up spot by the slimmest margin, finishing 0.01 behind Hoefl-Riesch.
World Cup leader Tina Maze of Slovenia missed the podium by 0.07 seconds but added 50 points to her overall World Cup tally, taking it to 1,744 points ahead of Hoefl-Riesch on 886.
France's world champion Marion Rolland had to be content with 10th place, 0.80 off the pace.
Maze and Hoefl-Riesch will be clear favourites in Sunday's super-combined on the same Roc de Fer piste, home to the women's races at the 1992 Olympics.