Amasa Stone, Jr. was born on April 27, 1818, on a farm near Charlton, Massachusetts, to Amasa and Esther ( Boyden) Stone. He was the ninth of 10 children, and the third of four sons. His ancestor, Gregory Stone, a yeoman, had emigrated from Ipswich in Suffolk, England, to Massachusetts in 1635 as part of the Puritan migration to New England. His great-grandfather, Jonathan Stone, fought at the Battle of Lexington on April 19, 1775, and in the subsequent American Revolutionary War.