The shooting happened about 2:15 p.m. at the Paper Source store when a man showed up and got into an argument with his ex-wife, police said.
He then shot and killed her before turning the gun on himself.
Police said the man, who was taken to the hospital in critical condition, is a 33-year-old resident of the Los Angeles area.
The woman, 30, lived in Thousand Oaks, though it is unclear whether she worked at the Paper Source.
The two had children, police said.
"Their children are fine. They've been located at this time," said Ventura County Sheriff's Sgt. Eric Boschow.
Mall shoppers who heard the gunfire panicked, thinking it was an active shooter situation, according to witnesses. Some ran out of the mall, while others hid in stores.
The entire mall was put in lockdown immediately after the shooting.
"The look on people's faces were just terrifying." said mall patron Kiana Mallas. "That's when we knew something was up, because their faces and their screams were just terrifying."
RELATED: A shopper talks about what it was like being in the mall in the aftermath of the shooting:
RELATED: Witnesses share video of the scene outside the mall as the situation unfolded: