Cox’s spokeswoman Nghia Demovic said the lawmaker died peacefully at his home in the Sacramento suburb of Fair Oaks.
Cox represented the sprawling 1st Senate District, a heavily Republican district that encompasses parts of 12 Sierra and foothill counties from the northeastern tip of California at the Oregon border to Mammoth Lakes in Mono County.
The death leaves two vacancies in the 40-member chamber. Republican Abel Maldonado left to become lieutenant governor in April.
Cox was elected to the Senate in 2004 after six years in the Assembly, where he once served as minority leader. He previously served six years as a Sacramento County supervisor.
He is survived by his wife of 44 years, Maggie, and three daughters.
“A devoted family man, he always found time to serve his community and constituents,” his family said in a statement issued through his Senate office. “Dave took great pride in public service and making government work for the people it serves.”
Memorial services were pending, his office said.