The first woman officer to be appointed director-general of the British Security Service, Rimington began to work for MI5 as a part-time secretary in Delhi when her husband was posted to the British High Commission and the local security liaison officer needed some clerical assistance. Upon her return to London, she joined the organization full-time and worked in every branch, making her an obvious choice to succeed Sir Patrick Walker upon his retirement in 1992. In 2001 she published her memoirs, Open Secret, in which she was fiercely critical of the male-dominated culture she had found in MI5 when she first joined.