John Campbell Merriam was born on October 20, 1869, in Hopkinton, Iowa, United States and was the eldest of three children of Charles Edward Merriam and Margaret Campbell Kirkwood. His only brother, Charles Edward Merriam, became a prominent political scientist at the University of Chicago. His mother, a schoolteacher, had been born in Pennsylvania but had grown up in Scotland. His father, of old American stock, had moved from Princeton, Massachusetts, to Iowa in the 1850s. After serving in the Civil War, he became a merchant and a political leader in Hopkinton, a Presbyterian elder, and a trustee of the local Lenox College. As a young man, Merriam began collecting Paleozoic invertebrate fossils near his Iowa home.