Daniel de Leon was born in the Dutch colony of Curacao on 14th December, 1852. His parents sent him to Germany and the Netherlands to be educated and he arrived in the United States in 1874. Soon afterwards he found work as a teacher of Latin, Greek and Mathematics.
De Leon settled in New York and became a student and later a teacher at Columbia University. Converted to socialism by the writings of Edward Bellamy, De Leon joined the Knights of Labor and worked for Henry George in his campaign to become mayor of the city.
In 1890 De Leon joined the Socialist Labor Party (SLP) and the following year became the editor of its newspaper, The People. In 1891 he ran as SLP candidate for governor of New York and won 13,000 votes.
Over the next few years De Leon, Laurence Gronlund, Morris Hillquit and Abraham Cahan emerged as the main leaders of the party. De Leon, a Marxist, began arguing for the revolutionary overthrow of capitalism. He claimed that as United States was the most developed capitalist country in the world it was "ripe for the execution of Marxian revolutionary tactics."
In 1892 the SLP Simon Wing ran for President, with Charles H. Matchett as Vice President. They received 21,173 votes. In 1896 the SLP's vote increased to 36,367 and in 1898 reached a peak of 82,204. At that time the party had 10,000 members.
De Leon was highly critical of the trade union movement in America. Originally a member of the Knights of Labor, In 1895 he helped form the Socialist Trade and Labor Alliance (STLA) as a revolutionary alternative to the conservative American Federation of Labour. The move was not very successful and after several years only had 20,000 members.
In the 1900 presidential election the Socialist Labor Party candidates received only 33,382. The other major left-wing party, the Social Democratic Party (SDP), led by Eugene Debs and Victor Berger, did better winning 97,000 votes.
On the 27th June, 1905, De Leon and a group of radical trade unionists held a convention in Chicago. Those who attended the proceedings included Eugene Debs, Bill Haywood, Mother Jones, Lucy Parsons and Charles Moyer. At the meeting it was decided to form the radical labour organization, the Industrial Workers of the World (IWW).
In 1908 the Wobblies, as they became known, split into two factions. The group headed by De Leon and Eugene V. Debs advocated political action through the Socialist Party and the trade union movement, to attain its goals. The other faction, led by Bill Haywood, believed that general strikes, boycotts and even sabotage were justified in order to achieve its objectives. Haywood's views prevailed and De Leon left the organization.
De Leon then went on to form the Workers' International Union in opposition to the Industrial Workers of the World. As the historian Paul Buhle has pointed out: "He was the first English-speaking intellectual to influence long-run trends in the American Left, Daniel De Leon could not build a successful socialist movement but he did elucidate powerful notions about the evolution of industrial society."
Daniel de Leon died in New York on 11th May, 1914. Over 30,000 people attended his funeral and his supporters remained dedicated to his principles and memory and reprinted his essays in every edition of Weekly People and circulated his pamphlets widely.