Ian Long is dead, and his body was found inside the bar, police said.
Dean said he has no doubt Helus and a California Highway Patrol officer who also was the first to respond "saved lives by going in there and engaging with the suspect".
"It's got to change", said Gavin Newsom, governor-elect of California, at a news briefing on Thursday.
A Ventura County Sheriff's Office official said around 30 shots were fired inside the building.
"It is with a profoundly heavy heart that I write this morning with regard to the tragic mass shooting that occurred during College Night at the Borderline Bar and Grill in Thousand Oaks last night".
The gunman was tall and wearing all black with a hood and his face partly covered, witnesses told TV stations.
A gunman opened fire around 11:20 p.m. PT at the crowded country-and-western bar about 65 kilometres northwest of Los Angeles, sending hundreds fleeing, including some who used barstools to break windows and escape.
Dean said that Helus "died a hero".
He said he has no doubt Helus' tactics at the scene "were sound when he went in". The gun was legally purchased, he added.
A gunman has opened fire at a bar in California killing at least 12 people, including a police officer. Police said they can not confirm a motive for the shooting.
Kuredjian said it has been "quite some time" since there was a shooting of any kind in Thousand Oaks, a city of about 130,000 people about 65 kilometres west of Los Angeles, just across the county line.
Cody Coffman, 22, was confirmed to have died during the attack by his father, Jason.
The Ventura County sheriff said Sgt. Ron Helus was talking with his wife when he found out about the shooting.
Officials have not released the names of all the victims as they worked to notify the families. The bar was hosting a college country music night and was packed with students and other patrons. Once safely outside, the uninjured hugged and awaited word about those still inside.
Twenty-one victims were treated at local hospitals and had been released by Thursday morning, according to the sheriff's office.
"Bar stools go through windows", he told ABC.
"You know, I've been a cop for 41 years, and I never thought I would see the things around the country that would happen", Dean said.
Long was on active duty with the Marines from August 2008 to March 2013, according to the Department of Defense.
"I can't tell you what the status of any of those that were shot is at this point", he said.
Police have not identified a motive.
As for why he attacked the bar, "there's no indication that he targeted the employees".
Dean said the shooter was already dead when officers made entry. "Multiple injuries reported. Details still being determined".
"Great bravery shown by police".
CBS LA reports that minutes after the shooting started, Helus was the first to enter the bar. He added that Helus was a "hard-working, dedicated Sheriff's Sergeant". "Thank you to Law Enforcement", the president said in another tweet.
This is a developing story, check back for updates.