Following the war, he was appointed military governor of California, serving from May 31, 1847, until April 13, 1849.
In the US Federal Census of 1850 for Jefferson Barracks, St. Louis County, Missouri, dated 18 August 1850, the following annotation is located at the bottom of the third page: "Brig Genl Mason died at Jefferson Barracks July 1849 of Cholera."[REF]