The crash happened at about 8:20 a.m. at a Baskin Robbins at 785 Oak Grove Road.
It appears the woman had just finished doing some morning shopping at Trader Joe's when she put her Toyota Corolla into drive instead of reverse, Contra Costa County Fire Protection District Battalion Chief Terry Stewart said.
The car drove up onto the sidewalk, took out a support beam that holds up the overhang in front of the business, and then went through the store's front window and crashed into the ice cream display cases, Stewart said.
A young woman was behind the counter at the time preparing to open the store for the day, and was trapped when the car came crashing through, he said.
"The employee was struck with some flying glass and had some minor injuries," Stewart said.
When fire crews arrived, the employee remained stuck behind the counter and the woman was still in her car inside the business, he said. Some firefighters began freeing the employee, while others started shoring up the sagging overhang.
"We were afraid that it was going to collapse and compromise the integrity of the building," Stewart said.
The employee was taken to a hospital to get checked out, and the driver was interviewed by Concord police, he said.
Stewart said much of the exposed ice cream will have to be thrown out, noting that some had shards of glass in it. "It's probably melting at this point," he added.
The car has been removed from the building and didn't appear to be badly damaged, he said.
The building's utilities were secured, and the property owner, the business owner and a city building inspector were called to the scene.
Stewart said power was briefly out at Flying Colors Comics next door, but store employee Andrew Bardin said the lights were back on before the shop was scheduled to open for the day.
Bardin said he had not spoken with the employee involved in the crash.
"We've been giving them their space because there's a lot of people going over there," he said.
The Baskin Robbins is located near Oak Grove Road and Treat Boulevard, the same intersection where 41-year-old Solaiman Nuri and his 9-year-old daughter Hadessa where struck and killed by an SUV driven by a local teen as they rode their bikes on April 7.
The speeding SUV went out of control, struck a fire hydrant, hit the bicyclists then careened into a building.
The building is being repaired this week.
"They just started rebuilding the wall yesterday," Bardin said. Stewart called it "uncanny" that today's crash happened "literally right across the street from where we had the tragedy on Easter weekend."
He said some of the same crews who were called to the scene of the April crash responded to today's incident.