Yi was born in Geoncheon-dong Street (건천동; 乾川洞), Hanseong (then capital, present-day Inhyeon-dong, Jung-gu District, Seoul) but spent his adolescence and early adulthood period before passing the military examination in Asan where his mother's relatives lived and where now a shrine to him stands.
As a young boy, Yi played war games with other local boys, showing excellent leadership talent at an early age and constructed his own bow and fletched his own arrows as a teenager.