Born and reared in Maine, Plummer went west in 1852 and settled in the California gold mining town of Nevada City. There he opened a bakery and became active in the Democratic Party. Well-spoken, friendly, and ambitious, Plummer won election to sheriff in 1856 and performed well enough to win re-election the following year. Later in 1857, however, Plummer suffered two setbacks. First, he lost his bid to win a seat on the California legislature. Then he was convicted of second-degree murder for killing an unarmed man in the line of duty. Plummer claimed he had acted in self-defense, but the jury apparently believed witnesses who said the true motive lay in an affair he was having with the murdered man's wife.