She became the personal assistant to the chief of the service in 1974 and later transferred to the SIS's department at the centre of the Cold War to support its operations behind the Iron Curtain.
La Fontaine accepted a posting at the British Embassy in Hanoi six years later, becoming the sole regular Western source in Communist Vietnam.
[1] She was the youngest of three children of army officer and colonial civil servant, Sydney Hubert La Fontaine, and Honor Kathleen Milton.
[1] In 1962, she flew to Kinshasa (formerly Léopoldville) to join her sister, Jean, and her husband, John Sackur of the Secret Intelligence Service (SIS), to assist them in a research project.
Although the SIS was male-dominated, there was a strong mutual trust that largely ignored age and rank and stemmed from undertaking lonely and demanding jobs, in which the full responsibility went to secretaries.
[2] In the Oxford Dictionary of National Biography, Gerald Williams wrote that she had "the right qualities for a role that called for intelligence, common sense, self-reliance, and the ability to impart confidence to those whose safety depended on her", making her "a good field operator.
She gained influence through her directness and humanity, and undertook appointments in personnel management, moving up the ladder to attain the rank of under-secretary.
From 1995 and 2000, she served as the chair of governors of John Donne Primary School in Peckham and also worked on the history of the Special Operations Executive as a research assistant.