Investigators listed "hypertrophic heart disease" as the cause of the 52-year-old's death on Feb. 4 at his home in Los Angeles. "Hypertrophic" means the heart muscle has become abnormally thick, making blood-pumping difficult.
Heavy alcohol use along with a congenital artery problem contributed to St. John's death, the report said.
Three days earlier, St. John had been released from a mental health hospital where he had been admitted for alcohol abuse and threatening self-harm, according to the report. It also listed a history of mental-health and alcohol problems.
He last spoke to someone about 24 hours before paramedics declared him dead in his apartment on a Sunday morning, the report states.
For 27 years, St. John played struggling alcoholic and ladies' man Neil Winters on the CBS soap opera, "The Young and the Restless."
He was nominated for 11 daytime Emmys, winning twice, for outstanding younger actor in a drama series, in 1992 and supporting actor in 2008.
He died four years after the death of his 24-year-old son, and St. John had spoken on social media about his struggles with grief.
He had become engaged to model Kseniya Mikhaleva in September.
St. John's last episode of "The Young and the Restless" aired in the week following his death. A story line that pays tribute to both Kristoff and his character is set to start in late April.