He is regarded as the forefather of the idea of revolutionary industrial unionism and was the leading figure in the Socialist Labor Party of America from 1890 until the time of his death.

While in Europe he had become fluent in German, Dutch, French, English, ancient Greek and Latin, in addition to his first language Spanish.

Some allege that the University officials denied him a promised full professorship because of his political activities,[7] while other believe that his subject was too esoteric to be a permanent part of the curriculum.

[8] De Leon published no papers about Latin America during this period, but he did contribute an article to the debut issue of the Academy's Political Science Quarterly on the Berlin West Africa Conference.

[9] He also wrote reviews on Franz von Holtzendorff's Handbuch des Völkerrechts in June 1888 and its French translation in March 1889 for the same publication.

After a traditional Jewish wedding in Caracas the family moved to a Spanish speaking area of Manhattan, at 112 West 14th street where their first son, Solon De Leon would be born on September 2, 1883.

After this the De Leons left the Lower East Side and moved in with their housekeeper Mary Redden Maguire at 1487 Avenue A.

He arrived on April 23 and was hosted by a 26-year-old school teacher, Bertha Canary, who was the head of a local Bellamyite group, the Christian Socialist Club.

Canary was familiar with De Leon, having read some of his articles in the Nationalist Club movement press, and the two apparently became infatuated with each other.

De Leon became a Marxist in the late 1880s, and argued for the revolutionary overthrow of capitalism, trying to divert the SLP away from its Lassallian outlook.

[14] Others question this assertion because by the same logic Marx and Engels could be described as advocates of the Iron Law because language in The Communist Manifesto and Value, Price and Profit pertaining to the level of wages and temporary effect of union activity on working conditions is similar to the language used by De Leon in his answer to Connolly, and the 'iron law of wages' is a Malthusian theory which De Leon did not indicate any support for.

At this early stage in De Leon's development, there was still a considerable remnant of the general unionist Knights of Labor in existence, and the SLP worked within it until being driven out.

De Leon was an important figure in the US labor movement, and in 1904 he attended the International Socialist Congress, held in Amsterdam.

[17] George Seldes quotes Lenin saying on the fifth anniversary of the revolution, "... What we have done in Russia is accept the De Leon interpretation of Marxism, that is what the Bolsheviki adopted in 1917.