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.