Mary Louise Prather joined the Signal Intelligence Service in 1938 and worked in the clerical field. After a few years she was promoted to Chief of the Stenographic Section. Her duties expanded to include logging cipher messages and preparing the office's decrypted messages for distribution. Due to her careful logging she identified a relationship between two Japanese messages that led to the recovery of a new code.
During WWII she was promoted to Administrative Assistant and Personnel Officer for the General Cryptographic Branch. In 1946 she received the Commendation for Meritorious Civilian Service. Through the 1950s she held a variety of positions increasing in responsibility until she was named the chief of the Soviet Information Division in 1960. In 1969 she received the Meritorious Civilian Service Award.