Marian Castle, née Johnson, was born in Kendall County, Illinois, a daughter of Rev. Oliver C. Johnson, a Presbyterian minister, and his wife Anna French Johnson, an author and lecturer. She spent her childhood in small parish towns in Illinois and Wisconsin. She began writing in childhood. She attended Carroll College in Wisconsin and Millikin University in Illinois. At age 16, she took a year out from her studies to teach in a county school in northern Wisconsin. In 1920, she received a Ph.B. (Bachelor of Philosophy) degree from the University of Chicago. Due to health problems, she moved to Albuquerque, New Mexico where she worked for the YWCA as a secretary and met her husband, Edward Carrick Castle. They married in 1924 and moved to Denver, Colorado. She began writing Western stories for magazines such as Ranch Romances and Ranchland Love, using the male pseudonym Monty Castle. In 1946, she published her first novel, Deborah. It was followed by Golden Fury (1949), Roxanna (1955), and Silver Answer (1960). Much of her writing focused on life in Colorado and the west in the late 19th century. She also contributed short stories, articles and essays to leading national magazines such as Harper's, Good Housekeeping, and Reader's Digest. She received numerous awards from the Colorado Authors' League.