In February of 1910, not long after having read the platform of Eugene V. Debs (1855–1926), John and Dolly Sloan became members of the Socialist Party, and, that fall, Sloan allowed his name to be placed on the ballot as a Socialist Party candidate for New York State Assembly. An excellent organizer, Dolly devoted herself to the party in many ways, even serving as secretary of Branch One, often called the silk stocking branch of the Socialist Party for its concentration of artists and intellectuals. Sloan did his part as an illustrator and designed posters and flyers for socialist rallies, lectures, and fundraisers. In the early twentieth century, socialist rallies in major American cities attracted thousands.
In 1910 Sloan began to create illustrations for socialist publications, including Coming Nation, Progressive Woman, Issue, and The Call. Often inspired by specific injustices the artist observed, Sloan's drawings for these socialist periodicals could be pointed in their messages. The fire that killed 146 of 500 employees at the Triangle Waist Company in March of 1911 was the subject of one of Sloan's strongest political illustrations. Sloan graphically symbolized the tragedy in the burned body of a young woman, which he surrounded with the triad of "Rent," "Profit," and "Interest" to implicate the larger forces involved.
In 1912, Dolly Sloan was involved in the controversial project to find temporary homes in New York for the children of striking workers from Lawrence, Massachusetts. The Lawrence textile workers' strike was a watershed in labor history. It was the first time the Industrial Workers of the World organized a massive strike of unskilled, foreign-born workers. The strike led to violence, arrests, and a congressional investigation into the condition of the mills.
The following year John Sloan was involved in another famous event in American labor history when he designed and produced sets for the Paterson Strike Pageant, an elaborate play staged at Madison Square Garden that illustrated the dire situation of striking silk workers in Paterson, New Jersey. The project brought together key figures from New York's artistic, literary, and political worlds, including Bill Haywood, leader of the I. W. W., the writer John Reed, and art patron Mabel Dodge Luhan. The pageant played to a standing-room–only crowd.
Much of Sloan's political activism was wrapped up in his involvement with The Masses, a socialist magazine which began publication in 1911. Sloan joined the editorial board in December 1912 and played a crucial role in creating a publication as committed to artistic freedom as it was to party politics. His contributions to The Masses were often pictures of scenes observed on the streets and in the parks of New York, and they weren't always clear in their messages. The image of giggling, healthy working-class girls in his July 1913 cover design, The Return from Toil, contradicts the idea of "toil" in the title and might have undermined socialist arguments about the exploitation of workers. Over time, The Masses became more entrenched in party politics, and the writers began adding captions to the artists' drawings, often to give them a more clearly socialist spin. In 1917 Sloan resigned the magazine in protest and left the Socialist Party. Dolly Sloan remained politically active even after they severed their ties with the party.
John Sloan always maintained the separation of his art and his politics, and though his paintings show his sympathy with the lower classes and distinct dislike of the privileged, Sloan's New York paintings, unlike his illustrations, rarely address political topics directly and specifically.