After her graduation in 1909 she traveled in Europe before she settled in Greenwich Village and was associated with a group of socialists involved in the production of The Masses journal. This included Max Eastman, John Reed, Floyd Dell, Crystal Eastman, Louis Untermeyer, William Walling, Art Young, Michael Gold, Boardman Robinson, Robert Minor, Randolph Bourne, Dorothy Day, Mabel Dodge, Mary Heaton Vorse and Louise Bryant.
On 3rd March 1913 Milholland led the women's suffrage demonstration in Washington on a white horse. Wearing white robes, the photograph of Milholland during the parade became one of the most memorable images of the struggle for women's rights in America. After one demonstration Milholland was said to be the "most beautiful woman ever to bite a policeman's wrist."
Inez was introduced to Boissevain by Max Eastman. Boissevain was a businessman who made his fortune by importing coffee beans from Java. A friend, Alyse Powers, argued that he was "handsome, reckless, mettlesome as a stallion breathing the first morning air, he would laugh at himself, indeed laugh at everything, with a laugh that scattered melancholy as the wind scatters the petals of the fading poppy...He had the gift of the aristocrat and could adapt himself to all circumstances ... his blood was testy, adventurous, quixotic, and he faced life as an eagle faces its flight." In July 1913 the couple were married.
Milholland, like most of the people associated with The Masses, believed that the First World War had been caused by the imperialist competitive system and that the USA should remain neutral. This was reflected in the fact that the articles and cartoons that appeared in journal attacked the behaviour of both sides in the conflict. In December, 1915, Milholland and other pacifists travelled on Henry Ford's Peace Ship to Europe.
On her return to the United States she became one of the leaders of the National Women's Party. The movement's most popular orator, Milholland was in demand as a speaker at public meetings all over the country. Milholland, who suffered from pernicious anemia, and was warned by her doctor of the dangers of vigorous campaigning. However, she refused to heed this advice and on 22nd October, 1916, she collapsed in the middle of a speech in Los Angeles. She was rushed to hospital but despite repeated blood transfusions she died on 25th November, 1916.
After her death the women's movement helped create the image of a martyr. A popular poster stated: "Inez Milholland Boissevain: Who Died for the Freedom of Women."