A native of Ganghwa, Lee Sun-seung linked up with Ro Tae-jun in October 1939 and used Kuanpo in Guandian County as a base to recruit youths and gather intelligence, joining the Korean Liberation Army's 2nd Detachment in May 1942. In October 1944 he escaped enemy-held territory with Gye Ui-seong and Song Chang-seok to rejoin the main unit, then received special OSS training with American forces and was assigned to the Gyeonggi Province team of the domestic advance army's 1st district. In 1945 he trained in wireless communications with the OSS radio unit and was awaiting infiltration into Korea with the Chungcheong Province team's 2nd district when liberation came.