Muir was born in the mining town of Rock Springs, Wyoming. She attended the University of Washington in Seattle, where she worked as the assistant editor of a student paper. After graduation, she briefly worked as a teacher before quitting to pursue a career as a newspaper reporter. She began her professional newspaper career at "The Salt Lake Herald" after convincing the city editor to break with tradition and hire their first female reporter. Eventually, she moved to "The Salt Lake Tribune" where she was, again, their first female reporter. After brief stints at other papers, she went to work for the "New York Daily News" as a police reporter in 1927.