Milly left Ukraine at the age of 17 for London in 1894. Once there she began working in a tailoring sweatshop in order to save up enough money for her parents and sisters to join her in the UK.

The harsh conditions she worked in, together with a baker's strike she took part in, led her to become involved with the group around the Jewish anarchist newspaper 'Arbeter Fraynd' ("Worker's Friend" in Yiddish). She became influenced by the works of the anarchist theorist Peter Kropotkin.

She met her partner Rudolf Rocker through her activism in 1895, and from October 1898 the pair co-edited 'Arbeter Fraynd'. In March 1900, the two also started publishing the newspaper 'Germinal', which was more focused on cultural topics.

After Milly's release from prison for her anti-war campaigning and Rudolf's deportation after internment as an enemy alien in England, the couple and their son Fermin moved to Berlin in November 1918.

Both Milly and Rudolf continued their activism and joined the 'Free Workers' Union of Germany' (FAUD), an anarcho-syndicalist trade union in 1919.

After its founding in early 1919, a discussion about the role of girls and women in the union started. The male-dominated organization had at first ignored gender issues, but soon women started founding their own unions, which ran separately butstill formed part of the FAUD.

Witkop was one of the leading founders of the 'Women's Union in Berlin' in 1920. On October 15, 1921, the women's unions held a national congress in Düsseldorf and the 'Syndicalist Women's Union' (SFB) was founded on a national level. Shortly thereafter, Witkop drafted 'Was will der Syndikalistische Frauenbund?' (What Does the Syndicalist Women's Union Want?) as a platform for the SFB. From 1921, the Der Frauenbund was published as a supplement to the FAUD organ Der Syndikalist, Witkop was one of its primary writers.

Milly reasoned that proletarian women were exploited not only by capitalism like male workers, but also by their male counterparts. She contended therefore that women must actively fight for their rights, much like workers must fight capitalism for theirs. She also insisted on the necessity of women taking part in class struggle, and that domestic work should be deemed equally valuable as wage labour.

In a 1921 article in 'Der Frauenbund', Witkop argued that the most important issue facing the SFB was the "sexual issue". She called for access to birth control and advocated a childbearing strike. A debate on the issue within the German syndicalist movement ensued. Meetings on the issue were well-attended meetings and new SFB chapters were formed.

She was not only active in the syndicalist and feminist movement, but also worked to fight racism and anti-Semitism. She was often frustrated by what she considered an unwillingness to fight anti-Semitism in the labour movement.

Milly and Rudolf fled Germany in 1933 to the United States, where they continued their political activism.