Orange County firefighters were called to 1101 E Imperial Highway to a report of two vehicles crashing into a building, resulting in several victims.
The crash occurred just after 4.10pm local time (10.10pm BST).
The SUV appeared to have crashed into the rear-end of the other vehicle, pushing the car in front inside the store.
The other vehicle ended on the pavement with its front end sticking through the front of the strip mall.
Police said at least six people were injured, including the 88-year-old female driver of one of the vehicles.
The woman was taken as a trauma patient to a local hospital.
Footage on social media shows an SUV crashing into the glass storefront, followed by a smaller vehicle behind it.
Police believe the other injuries are not life-threatening.
Placentia Police wrote on Twitter: “All minor injuries with one transported to a local hospital.
“Unknown cause at this time.”