The victim, 30-year-old Sergio Mendez, was an Iraq war veteran who had enlisted in the National Guard.
The crash happened shortly after 7 p.m. Authorities said Michael Cedarland, who has been with the sheriff's department for seven years, lost control of his truck and traveled about 300 feet from the road into the building in the 600 block of Ventura Street.
The car broke through the east wall of the store, plowed through the customer lobby area and came to rest at the front door.
Mendez was pinned between the vehicle and a wall, according to investigators.
Authorities said drugs or alcohol do not appear to be a factor in the collision. It was unclear if Cedarland will face any charges.
Vishva Bandhu was working at the pizza place next door at the time and said the crash felt like an earthquake.
"When we came out we saw a guy lying down on the floor and he was stuck inside the wall and the big truck," he said.
Bandhu said one woman tried to help Mendez but it was too late. He says the woman was holding up Mendez's head to give him support, but the veteran was unconscious.
Firefighters had to use the Jaws of Life to get Mendez out. He sustained multiple internal injuries and was transported to Ventura County Medical Center. He succumbed to his injuries during emergency surgery.
"What I just don't understand is that he came back from his war to die here in Fillmore. That's unbelievable. I'm going to miss him," said Richard Gonzalez, a friend of Mendez.
Two other customers, Jose Iraheta of Lancaster and Thomas Michaels of Piru, were treated for minor injuries and released.
Cedarland, 30, was also taken to the hospital due to the strong impact of the crash, but he was not injured.
Authorities said Cedarland was on his way to visit a friend going westbound on Highway 126 when he fell asleep at the wheel.
The impact of the crash was so strong that officials had to red-tag the building. The cause of the crash remains under investigation.