Little is known of Marion Kingsley beyond the information given in the letters. She was evidently unmarried ("Miss Kingsley") and went to Korea in 1954 to work at the Methodist Mission in Kangneung (Kangnŭng-si) in South Korea. Her work included setting up schools, hospitals and clinics. She was likely a nurse, as she mentions teaching nursing courses locally and at Yonsei University and Seoul National University.
From the guide to the Marion Kingsley Letters, 1955-1990, (Special Collections Research Center, Syracuse University Libraries)