The extent of the victim's injuries has not yet been released.
The sheriff's spokesman says the first reports of shots fired came around 11:20 p.m. local time at the Borderline Bar & Grill in Thousand Oaks, which is about 64 kilometres west of Los Angeles.
Police also said it was an ongoing incident with an active shooter on spot.
Officers at the scene told the newspaper that about 30 shots had been fired.
Multiple people were seen being led out of the bar by deputies.
The suspect reportedly threw smoke bombs inside the bar.
Hedge said he managed to run out the front door as the shooter made his way to the back of the restaurant.
'The gunman had glasses and a black jacket.
Police have described it as a "mass casualty incident" and helicopters are swirling overhead.
They said that soon after the gunman opened fire, victims were running to a nearby petrol station to get help.
Officers escorted large groups of people to safety as they left the area.
A witness told KABC-TV that there were many young people "having a great time" in the bar when a man entered and "started shooting". A "College Country Nights" event was being held on Wednesday evening, according to the bar's website.
Borderline is a country music bar that hosts live music events.