John Mattocks spent his formative years in Vermont, where his father held a prominent position as the Vermont State Treasurer. He pursued a legal career, gaining admission to the bar and subsequently engaging in various political roles. Mattocks served in the Vermont House of Representatives and held a significant military position as a brigadier general during the War of 1812. He transitioned to federal politics, serving multiple terms in the U.S. House of Representatives and chaired the House Committee on Expenditures in the Department of War. Mattocks also served as a judge on the Vermont Supreme Court and participated in the State constitutional convention.

John Mattocks's political career reveals a deeply rooted commitment to fiscal responsibility and regional economic development, combined with a surprising advocacy for judicial reforms. His military service and emphasis on national defense juxtapose with a focus on local infrastructure, showcasing a dual dedication to both state and national interests. Mattocks's tenure in the Vermont House and U.S. House, along with his judicial role, highlights an ideologically agile approach to governance, navigating between local and federal responsibilities. His personal tragedy, marked by his son's suicide, adds a layer of complexity to his political persona, hinting at a nuanced understanding of the human cost of public service.