Born in rural Gibson County, Indiana, Johnson never attended University; while born to a prominent family, his plans for his studies were foiled by economic effects of the Panic of 1837. He apprenticed a printer before moving to Iowa to work with a lawyer, and was admitted to the Iowa Bar. In July 1849, Johnson left Iowa for the Gold Rush in California, where he briefly employed himself as a gold prospector, and later as a mule train driver. Johnson restarted his law career in Sacramento, California by founding a law practice with Ferris Forman, and was elected as Sacramento City Attorney in 1850. After two years in the City Attorney's office, Johnson began his political career by running as a Whig in the 1852 election, in which Johnson was elected to the California State Assembly as one of four members representing Sacramento.