Catherine O'Hara died from a pulmonary embolism, with cancer as the underlying cause, according to her death certificate.
The certificate, released on Monday, lists the pulmonary embolism, which occurs when a blood clot blocks an artery in the lungs, as the immediate cause of the actor's death at age 71.
Rectal cancer was the long-term cause.
The oncologist who signed off on the certificate indicated that he had been treating O'Hara since March last year, and last saw her a few days before she died at a Californian hospital.
The Canadian-born comic actor notably starred as Macaulay Culkin's mother in two Home Alone movies, and won an Emmy as the dramatically oblivious, formerly-wealthy matriarch Moira Rose in Schitt's Creek.
She most recently starred as former studio head Patty Leigh in 10 episodes of Seth Rogen's award-winning TV comedy The Studio.
Her death came as a surprise to most, and an initial statement from her representatives shared only that she had died "following a brief illness".
Collaborators including Culkin, Rogen, Christopher Guest, Eugene Levy and Pedro Pascal paid her loving tributes after her death.
The document stated that O'Hara's body had been cremated. It lists her profession as actress and her business as movies.