Spargo became a socialist after reading the work of Henry Meyers Hyndman. He was especially impressed by England for All (1881), where he attempted to explain the ideas of Karl Marx. In 1896 Spargo formed a branch of Hyndman's Social Democratic Federation (SDF). Other members since its formation included Tom Mann, John Burns, Eleanor Marx, William Morris, George Lansbury, Edward Aveling, H. H. Champion, H. H. Champion, Guy Aldred and Ben Tillet.
Spargo was elected president of the Barry Trades and Labour Council and became a member of the National Executive Committee of the SDF. Markku Ruotsila, the author of John Spargo and American Socialism (2006) has argued: "It was an amazing, meteoric progression for an uneducated stonemason from Western Cornwall that took place in these few years of Spargo's education in Marxism... he was recognized as one of the most promising and energetic Marxist agitators in the country."
Although Henry Meyers Hyndman was a talented writer and public speaker, many members of the Social Democratic Federation questioned his leadership qualities. Hyndman was extremely authoritarian and tried to restrict internal debate about party policy. Several members including William Morris, Edward Aveling, Eleanor Marx John Burns and Tom Mann left the party. However, Spargo remained loyal to Hyndman.
On 27th February 1900, Hyndman, Spargo and the Social Democratic Federation met with the Independent Labour Party, the Fabian Society and trade union leaders at the Congregational Memorial Hall in Farringdon Street. After a debate the 129 delegates decided to pass the motion to establish "a distinct Labour group in Parliament, who shall have their own whips, and agree upon their policy, which must embrace a readiness to cooperate with any party which for the time being may be engaged in promoting legislation in the direct interests of labour." To make this possible the Conference established a Labour Representation Committee(LRC).
Soon afterwards Spargo was invited to go of a lecture tour of the United States. He took with him his new wife, fellow socialist, Prudence Edwards. The couple arrived in New York City in February 1901. Spargo saw the potential of the country and decided not to return to England. He became friends with Morris Hillquit. Spargo went to work for Hillquit but spent most of the time lecturing on socialism.
Later that year the Social Democratic Party (SDP) merged with Socialist Labor Party to form the Socialist Party of America. Spargo was one of its founding members. Leading figures in this party included Eugene Debs, Victor Berger, Ella Reeve Bloor, Emil Seidel, Daniel De Leon, Philip Randolph, Chandler Owen, William Z. Foster, Abraham Cahan, Sidney Hillman, Morris Hillquit, Walter Reuther, Bill Haywood, Margaret Sanger, Florence Kelley, Rose Pastor Stokes, Mary White Ovington, Helen Keller, Inez Milholland, Floyd Dell, William Du Bois, Hubert Harrison, Upton Sinclair, Agnes Smedley, Victor Berger, Robert Hunter, George Herron, Kate Richards O'Hare, Helen Keller, Claude McKay, Sinclair Lewis, Daniel Hoan, Frank Zeidler, Max Eastman, Bayard Rustin, James Larkin, William Walling and Jack London.
Spargo's first wife, Prudence, died of tuberculosis in March 1904. The following year he married Amelia Rose Bennetts, a British-born socialist who worked in a carpet mill. The couple had two children, a daughter named Mary and a son who died in childhood. According to John Patrick Diggins: "It was well known that on many of his trips Spargo cavorted with a number of attractive ladies, and he quickly built a reputation not just as an effective socialist organizer but as a womanizer of some note."
In September 1905, Spargo helped to establish the Intercollegiate Socialist Society. Other members included Jack London, Clarence Darrow, Florence Kelley, Anna Strunsky, Bertram D. Wolfe, Jay Lovestone, Upton Sinclair, Rose Pastor Stokes and J.G. Phelps Stokes. Its stated purpose was to "throw light on the world-wide movement of industrial democracy known as socialism."
In 1905 Spargo published the best-selling expose of slum life, The Bitter Cry of Children. This was followed by a book about child nutrition, Underfed School Children. In 1907 he published an account of his Christian Socialism, entitled, The Spiritual Significance of Modern Socialism. He was now considered one of the most important popularizers in the socialist movement. In 1910 Spargo published the first full-length biography of Karl Marx. The book, Karl Marx: His Life and Work, according to Robert Asher, "depicted the founder of scientific socialism as sentimental, but above all a pragmatic tactician".
Over the next few years Spargo became a controversial figure in the Socialist Party of America. Although he argued in favour of women's suffrage and civil rights for African Americans, he called for the restriction of immigration. Spargo held that this policy would make socialism more appealing to trade union members. He also strongly attacked the Industrial Workers of the World for advocating a general strike, a move that he considered to be "inflammatory, inviting employer and state repression".
On the outbreak of the First World War most socialists in the United States were opposed to the conflict. They argued that the war had been caused by the imperialist competitive system and argued that the America should remain neutral. In an article in September 1915 Eugene Debs wrote: "I am not opposed to all war, nor am I opposed to fighting under all circumstances, and any declaration to the contrary would disqualify me as a revolutionist. When I say I am opposed to war I mean ruling class war, for the ruling class is the only class that makes war. It matters not to me whether this war be offensive or defensive, or what other lying excuse may be invented for it, I am opposed to it, and I would be shot for treason before I would enter such a war."
After the USA declared war on the Central Powers in 1917, Spargo argued for a pro-war policy. He was supported by William Walling and Upton Sinclair, but when he was defeated at a special conference of the Socialist Party of America, he resigned from the party.
Spargo now moved to the right and became a member of the Republican Party, supporting Calvin Coolidge in the election of 1924 Presidential Election and Herbert Hoover in the 1928 Presidential Election. Spargo was also a strong opponent of Franklin D. Roosevelt and the New Deal. He lost interest in politics and devoted his time to the Bennington Museum in Vermont.
John Spargo died in 1966.