Johnson, heir to the Johnson & Johnson cosmetics company, was found dead at her Los Angeles home on 4 January (10) at the age of 30.
An initial autopsy on her body showed there were no signs of trauma and her cause of death was deferred, pending toxicology tests.
The Coroner's Office has now determined "diabetic ketoacidosis", caused by a lack of insulin, which can lead to a diabetic coma, was the cause of her demise.
According to TMZ.com, Johnson - who had a history of diabetes - was hospitalised at least twice before her death for diabetic comas.
Johnson's adopted three-year-old daughter Ava is currently being looked after by the socialite's mother Sale, 60, and her husband Ahmad Rashad in New York.