The coroner's office said Kinkade died April 6 due to a combination of alcohol and Valium.
Before his death, the 54-year-old self-described "Painter of Light" produced scenes of country gardens and pastoral landscapes.
In recent years, Kinkade had run into personal difficulties, including a 2010 bankruptcy filing by one of his companies. Kinkade's brother Patrick acknowledged recently that the painter had relapsed into alcoholism shortly before his death.
The Associated Press contributed to this report.