Her most successful books were A Man Called Peter (1951) and her novel, Christy (1967), which was inspired by the story of her mother's time in the mountains teaching the impoverished children of Appalachia. They include edited collections of Peter Marshall's sermons and prayers, and her own inspirational writings. Marshall wrote or edited more than 30 books, which have sold over 16 million copies. Her success encouraged her to keep writing. It became a nationwide success and was adapted as a film of the same name, released in 1955. Marshall wrote a biography of her husband, A Man Called Peter, published in 1951. He later also became a minister and author. Her husband died in 1949 of a heart attack, leaving her to care for their 9-year-old son, Peter John Marshall. She spent nearly three years recovering from the illness.

In 1940, Marshall contracted tuberculosis, for which at that time there was no antibiotic treatment. The couple moved to Washington, DC, where her husband served as pastor of the New York Avenue Presbyterian Church and Chaplain of the United States Senate.

While a junior at Agnes Scott College, she met Peter Marshall, marrying him in 1936. From the age of nine until her graduation from high school, Marshall was raised in Keyser, West Virginia, where her father served as pastor of a Presbyterian church from 1924 to 1942.

She was the daughter of the Reverend John Ambrose Wood and Leonora Whitaker Wood. Marshall was born in Johnson City, Tennessee.

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