Born in 1824 in Ireland, McClatchy was a young journalist on the editorial staff of Horace Greeley’s New York Tribune in 1848, when news of a gold strike on Northern California's American River reached the East.
He married Charlotte Maria McCormick (1829–1916) and had at least four children: Fannie, Emily Estelle, Charles Kenny and Valentine Stuart McClatchy.
Returning to journalism, he took a position in the summer of 1849 with the Placer Times, which was published at Sutter's Fort, the settlement that gave rise to the river port town of Sacramento, California.
As an editorialist, McClatchy developed a reputation as a people's champion after he took a stand against land speculators in what would evolve into the 1850 Squatters' Riot.
Known as a supporter of the people's interests against those of corporations and corrupt politicians, McClatchy made The Bee a bastion of progressive reformism.