Born Margaret Dorothea Mortenson to Anenus Duabus "A.D." and Adelle Mortensen in Somers, Wisconsin, she was one of three daughters in a devout Methodist family. The family moved to Evanston, Illinois, where she graduated from Evanston Township High School in 1921.

She attended Wheaton College in Wheaton, Illinois, graduating in 1925. She taught school for a year, then married Kenneth Landon, whom she knew from Wheaton, and in 1927 they signed up as Presbyterian missionaries to Thailand.

Between 1927-1937, Landon raised her three children while running a mission school in Trang and read extensively about the country. During her readings, she learned about Anna Leonowens, the late-19th Century governess to the Siamese royal family. When the Landon family returned to America in 1937, she soon began writing articles and then began researching material for a book on Leonowens.

The family moved to Washington, D.C., in 1942 when Kenneth Landon joined the United States Department of State as an expert adviser on Southeast Asia. In 1947, Kenneth authored Southeast Asia, Crossroad of Religion, subsequently reprinted in 1969 and 1974. He also wrote The Chinese in Thailand and Siam in Transition.

Landon's fourth child, Kenneth, Jr., was born in Washington, D.C. He followed the lead of his parents and took up writing about his own field of interest, releasing God of Glory: The Promise of Relationship in 1992.

Margaret Landon was married 67 years. She died in Alexandria, Virginia, December 4, 1993, aged 90, leaving 13 grandchildren and 25 great grandchildren. She is interred in Wheaton Cemetery in Illinois.