Conness moved to California in 1849 and became involved in mining. A member of the Democratic Party, Conness served as a member of the State Assembly (1853-1854 and 1860-1861) but failed in his bid to become governor of California in 1861. Soon afterwards he joined the Republican Party.
Elected to the 39th Congress in 1862 he was chairman of the Committee on Mining (1863-67). In their book, The Lincoln Conspiracy (1977), David Balsiger and Charles E. Sellier claimed that Conness was involved in the assassination of President Abraham Lincoln. John Conness retired from politics in 1869 and died in an insane asylum in Jamaica Plain, Massachusetts, on 10th January, 1909.