The second eldest daughter of Jakobus and Susan September, Dulcie grew up in Gleemore, a suburb in Cape Town, where she developed her interest in political activism. She began her primary schooling at Klipfontein Methodist Mission, and later attended Athlone High School. In 1954, she enrolled at the Wesley Training School in Salt River to pursue a career in teaching, and completed her Teacher's Diploma in 1955. She began her teaching career, first at City Mission School in Maitland, then at Bridgetown East Primary School in Athlone in 1956, and in 1957 became a member of the newly established Cape Peninsula Students' Union (CPSU), affiliate of the Unity Movement of South Africa, which aimed at overcoming racial divisions and forging solidarity among students of different cultural backgrounds. She belonged to the Athlone branch of the Teacher's League of South Africa (TLSA).
September subsequently joined the African Peoples Democratic Union of Southern Africa (APDUSA), established in 1960. She went on to be a member of the militant study group Yu Chi Chan Club, which was disbanded at the end of 1962, to be replaced by the National Liberation Front (NLF) in January 1963. While engaged in NLF activities, she was arrested and detained without trial at Roeland Street Prison on 7 October 1963. Together with nine others she was charged under the Criminal Procedure Act, the principal charge being "conspiracy to commit acts of sabotage, and incite acts of politically motivated violence". After months of court proceedings, judgment was delivered on 15 April 1964. September was sentenced to five years imprisonment, during which time she endured severe physical and psychological abuse. On her release in April 1969, the Pretoria regime controlled her activities with a five-year banning order, which prohibited her from engaging in political activity and from practising her profession. September then went to live with her sister in Paarl.
In 1973, as her banning order drew to a close, September applied for a permanent departure permit, having secured a position at Madeley College of Education in Staffordshire. She left South Africa on 19 December 1973. In London, she joined the activities of the Anti-Apartheid Movement and was in the frontline of numerous political rallies and demonstrations at South Africa House in Trafalgar Square. Later she gave up her job as a teacher and joined the staff of the International Defence and Aid Fund for Southern Africa. In 1976 she joined the African National Congress (ANC) where she worked in the ANC Women's League. In 1979, International Year of the Child (IYC), she was elected chairperson of the IYC Committee of the ANC Women's Section in London. At the end of 1983, September was appointed ANC Chief Representative in France, Switzerland and Luxembourg.
On the morning of 29 March 1988, September was assassinated outside the ANC's Paris office at 28, Rue des Petites-Ecuries, as she was opening the office after collecting the mail. She was shot five times from behind with a 22-calibre silenced rifle. She was 52 years old.
Before her assassination, September had been investigating trafficking of weapons between France and South Africa. Supposedly, this trafficking included nuclear materials. On the day after her murder, Alfred Nzo commented: "If ever there was a soft target, Dulcie September was one."
Jean Michel Jarre composed a song for his 1988 Revolutions album named "September", dedicated to Dulcie. The song was performed at his Destination Docklands concert at London's Royal Victoria Dock in October 1988.
The Conceptual Artist Hans Haacke devoted his piece "One Day, The Lions of Dulcie September Will Spout Water in Jubilation" to Dulcie September. The site-specific intervention that modified an existing but defunct fountain in front of the Grande Halle de la Villette in Paris, was part of the exhibition Magiciens de la terre by Jean-Martin Hubert.
A square in the 10th arrondissement of Paris is named after Dulcie September, and was officially inaugurated on 31 March 1998, ten years after her death. A street in Cléon, near Rouen is also named after her.
In October 2011, Staffordshire University Students’ Union honoured Dulcie September by renaming their boardroom the "September Room" and erecting a plaque in her memory. Dulcie September was a former student of Madeley College of Education, one of the founding colleges of North Staffordshire Polytechnic.
- "The Case Of 'Dulcie September'". The Truth Commission Files. Retrieved 28 May 2013.
- "Dulcie September: A dedicated cadre cut down by act of cowardice". ANC Today 2 (34). 23 August 2002.
- "Remembering 25 Years On". Retrieved 6 December 2013.