[3][4] In 1851, he opened a law practice in Napa, and was elected in 1852 as judge of the Seventh Judicial District Court for a term of six years, and in 1858 was re-elected.

[9] In August 1863, during the American Civil War, he was nominated for Lieutenant Governor by the Copperhead Democratic Party, but lost the election.

In October 1867, during the election, a Catholic priest accused McKinstry of leading the Aurora Vigilance Committee, a quasi-lynch mob, in 1863 in Esmeralda County, Nevada, and refusing to accommodate the request for last rites by four men who were condemned to hang.

[20][21][22] McKinstry filled the seat of former Chief Justice Royal Sprague, who died in office, and whose appointed successor, Isaac S. Belcher, did not run for election.

[27] On December 20, 1899, McKinstry delivered the oration to the Jubilee Celebration at San Jose, California commemorating 50 years of statehood.

[29] On July 27, 1863, McKinstry was married to Annie L. Hedges at Marysville, California, and they had two sons and two daughters: Capt.