Harrewijn was a preacher, who was a member of the Communist Party of the Netherlands. He was strongly committed to helping people on the lowest ranks of social society: the homeless, those on social security and addicted people. He was active in the CPN at the municipal and provincial level.
On February 1 1995 Harrewijn became chair of the GreenLeft which had formed in 1991 out of the CPN, the progressive Christian Evangelical People's Party and Political Party of Radicals and the left-socialist Pacifist Socialist Party. He remained at that position until the 1998 elections when he was elected in the Tweede Kamer. Here he focused on defense and social affairs. This earned him the nickname "general Ab". As the defense spokesperson of the GreenLeft he was a member of the temporary committee on decision-making procedures for military operations.
Harrewijn ran again in the 2002 elections, two days before the elections and a week after the murder on Pim Fortuyn Harrewijn died of the result of a stroke four days earlier. The Leftwing Cheek, the GreenLeft platform for religion and politics, instituted an Ab Harrewijn Price in his honour, for people who undertook a creative, small initiative for people on the lowest ranks of society. The National Client Council organizes a yearly lecture in his honour. In 2006 the Law Harrewijn was adopted. The law was based on an iniative of Harrewijn and fellow GreenLeft MP Paul Rosenmöller, which seels to inform works council on the salaries of the top levels of management and its Board of directors.