Haggerty was converted to Marxism before his ordination in 1892 and was later influenced by anarcho-syndicalism. His formal association with the church ended when he was suspended by his archbishop for urging miners in Colorado to revolt during his tour of mining camps in 1903. Shortly after, he became the editor of the American Labor Union's newspaper, the Voice of Labor.
Haggerty is credited with authoring the IWW Preamble, assisting in writing the Industrial Union Manifesto and drawing up the chart of industrial organization ("Father Haggerty's Wheel").
Haggerty favored direct action, as opposed to political action of the socialist political parties (he referred to them as slowcialists). During an IWW convention speech he said "The Ballot Box is simply a capitalist concession. Dropping pieces of paper into a hole in a box never did achieve emancipation of the working class, and in my opinion it never will."