Soon before Marx died in 1883, he wrote a letter to Lafargue and the French Workers' Party organizer Jules Guesde, both of whom already claimed to represent "Marxist" principles.

It was there that Lafargue started his intellectual and political career, endorsing Positivist philosophy, and communicating with the Republican groups that opposed Napoleon III.

Nevertheless, he soon began communicating with two of the most prominent revolutionists: Marx and Auguste Blanqui, whose influence largely ended the anarchist tendencies of the young Lafargue.

In 1865, after participating in the International Students' Congress in Liège, Lafargue was banned from all French universities, and had to leave for London in order to start a career.

[8] Lafargue was chosen as a member of the General Council of the First International, then appointed corresponding secretary for Spain, although he does not seem to have succeeded in establishing any serious communication with workers' groups in that country—Spain joined the international congress only after the Cantonalist Revolution of 1868, while events such as the arrival of the Italian anarchist Giuseppe Fanelli caused it to be influenced strongly by anarchism (and not the Marxism that Lafargue chose to represent).

Lafargue became involved with propagating Marxism, an activity that was directed largely by Friedrich Engels, and one that became intertwined with the struggles that both tendencies had internationally—as the Spanish federation of the International was one of the main endorsers of the anarchist group.

At the same time, Lafargue took initiative through some of his articles, expressing his own ideas about a radical reduction of the working day (a concept which was not entirely alien to the original thought of Marx).

Thanks to Engels' assistance, he again began communicating with the French workers' movement from London, after it had started to regain popularity lost as a result of the reactionary repression under Adolphe Thiers during the first years of the Third Republic.

[10]: 41 Nevertheless, Lafargue continued his defence of Marxist orthodoxy against any reformist tendency, as shown by his conflict with Jean Jaurès, as well as his refusal to participate with any "bourgeois" government.

During these later years, Lafargue had already begun neglecting politics, living on the outskirts of Paris in the village of Draveil, limiting his contributions to a number of articles and essays, as well as occasional communication with some of the better-known socialist activists of the time, such as Karl Kautsky and Hjalmar Branting of the older generation, and Karl Liebknecht or Vladimir Lenin of the younger generation.

[12] In his suicide letter, Lafargue explained:[13] Healthy in body and mind, I end my life before pitiless old age which has taken from me my pleasures and joys one after another; and which has been stripping me of my physical and mental powers, can paralyse my energy and break my will, making me a burden to myself and to others.

[...] [I]n Lafargue were two different aspects that made him appear in constant contradiction: affiliated to socialism, he was anarchist communist by intimate conviction; but enemy of Bakunin, by suggestion of Marx, he tried to damage Anarchism.

I recall that I vehemently objected to August Bebel, who was indignant over these suicides, that if one could argue against the age at which the Lafargues chose to die — for here we were dealing not with the number of years, but with the possible usefulness of a political figure — then one could by no means argue against the very principle of a political figure departing from this life at the moment when he felt that he would no longer bring any benefit to the cause to which he had devoted himself.Vladimir Lenin, who was one of the speakers at the funeral as representative of RSDLP,[14] later told his wife Nadezhda Krupskaya:[15] If one cannot work for the Party any longer, one must be able to look truth in the face and die like the Lafargues.