Seth Iredell Cadwallader was born in Milton, PA on December 5, 1874. He was named after Seth Iredell, a cousin, business partner and fellow Quaker of his grandfather, Seth Cadwallader. Iredell moved to Ohio in the late 1820s.
Seth was the third of eight children of Albert and Anna (Supplee) Cadwallader. He graduated from Jefferson Medical College in 1900 and became a physician. In 1901 he married Marian Isabella Koons, and they settled near Harrisburg, PA, where he maintained his medical practice until his death in 1957.