Johannes Guillaume Christianus Andreas Marijnissen was born on 8 October 1952 in Oss in the Dutch Province of North Brabant in a Roman Catholic family.

The SP's council representation in Oss has grown at every local election and is, as of 2005, the biggest stronghold of the Socialist Party in the Netherlands.

Nicknamed "The Wizard of Oss", after his hometown, he became famous when he turned out to be one of the main architects of the Dutch vote that rejected the European Constitution in June 2005.

Beginning in 1994, when the SP made the breakthrough which brought it into the House of Representatives, Marijnissen was chairman of the party's parliamentary group.

After the 2004 elections a conflict arose between Jan Marijnissen and Ali Lazrak, when the latter refused to conform to an agreement that required all SP politicians to donate part of their wages.

On 17 June 2008 Marijnissen announced his resignation as chairman of the SP group in the House of Representatives because of health reasons.

His appeal is also explained by some observers by his capacity of presenting himself as a valid, more social alternative to the PvdA, which moved into a more centrist third way direction by advocating welfare reforms under the leadership of Prime Minister Wim Kok and his so-called Purple Cabinets (1994–2002).