The firefighter was identified as Cal Fire San Diego Unit Fire Apparatus Engineer Cory Iverson, according to Cal Fire Director Chief Ken Pimlott.
Pimlott said Iverson was 32 years old and is survived by his wife, Ashley, and 2-year-old daughter Evie. Iverson's wife is currently expecting their second daughter, Pimlott said.
Iverson had been with Cal Fire since 2009.
"While we continue to process this tragic loss, we must keep our focus on the fire," Pimlott said. "The firefight in front of us continues to go on. The communities we are protecting are depending on us, and we will not fail."
Pimlott said Cal Fire has initiated a serious accident review team to look into Iverson's death. Pimlott said no further information could be shared until the investigation was complete.
Cal Fire San Diego Chief Tony Mecham's voice was heavy with emotion as he spoke of Iverson Thursday after his death.
"Cory's just a great young man, and, you know, he's a great young man. He's somebody who loved this job. He took great pride in wearing the Cal Fire patch. He was an extraordinary firefighter," Mecham said.
Dozens of fellow firefighters lined up in Fillmore to honor Iverson in a procession. Engines lined bridges above the freeway where Iverson's body was being transported from the Fillmore area to the Ventura County Coroner's Office.
California Gov. Jerry Brown issued a statement in the wake of Iverson's death, saying, "Anne and I are saddened by Engineer Cory Iverson's tragic death. His bravery and years of committed service to the people of California will never be forgotten."
Ventura County sheriff's officials said the firefighter suffered fatal injuries near the eastern flank of the fire sometime between 9 a.m. and 10 a.m. Thursday. The department's major crimes division is investigating the death.
Iverson had been fighting the Thomas Fire since it first erupted last week.
According to Cal Fire, the Thomas Fire has scorched 249,500 acres since it broke out Dec. 4, which amounts to more than 379 square miles. It is now the fourth largest wildfire in California history.
The colossal blaze, which straddles coastal Ventura and Santa Barbara counties, was 35 percent contained as of Thursday evening.
I am very saddened to report that a firefighter fatality has occurred on the Thomas Incident. Please join me in keeping our fallen firefighter and his loved ones in your prayers. - @CALFIRE_CHIEF https://t.co/lJjvOddchS pic.twitter.com/zylqSlsj3K— CAL FIRE (@CAL_FIRE) December 14, 2017