Daniel DeLeon (1852-1914) was the leading figure in the Socialist Labor party from 1891 until his death. Born in Curaçao, he was educated in Germany in the late 1860s and studied medicine in Amsterdam until 1872 without completing his course of study.

Between 1872 and 1874 he immigrated to the United States, and worked as a schoolteacher in Westchester, N.Y., until he entered the Columbia College School of Law in 1876. He earned his law degree in 1878 and practiced in Brownsville, Tex., until 1882 and in New York City from 1882 until at least 1884. From 1883 to 1889 he was a lecturer at the Columbia College School of Political Science and was active in reform and radical movements.

DeLeon worked in Henry George's 1886 mayoral campaign, joined Knights of Labor Local Assembly 1563 in 1888, and was

active in the utopian socialist Nationalist movement from 1889 to 1890, when he joined the Socialist Labor party. He ran for governor of New York on the party's ticket in 1891.

DeLeon became editor of the SLP's journal, the People, in 1891 a post he held until shortly before his death. In 1891 he was elected a delegate to KOL District Assembly 49, and he played a prominent role in the KOL until 1895, when he launched an alternative labor federation, the Socialist Trade and Labor Alliance. He was active in the IWW from 1905 to 1908, when he broke with that organization after a struggle with its syndicalist faction. Although DeLeon's supporters withdrew and set up a second IWW in Detroit, he discouraged the establishment of the new organization and was never active in it.