Thom arrived in California in 1849 during the gold rush and after a few years of successful mining, he studied law in Sacramento. In fall 1853 he moved to San Francisco, where he worked as a deputy agent for the United States Land Commission, and then to Los Angeles, where he had a similar job. Shortly after arriving, he was appointed Los Angeles County district attorney, and he later won the office in an election. He was also elected Los Angeles city attorney for the 1856–58 term.