 Mattocks was born in Hartford, Connecticut on March 4, 1777, and moved with his parents to Tinmouth, Vermont in 1778.His father, Samuel Mattocks, was a veteran of the American Revolution and served as Vermont State Treasurer from 1784 to 1800.John Mattocks pursued an academic course, studied law in Middlebury, Vermont and Fairfield, Connecticut, and was admitted to the bar in 1797.