A teacher and journalist, during his youth he was involved in various political movements until he joined the anarchist Confederación Nacional del Trabajo (CNT).

[2] Despite his modest origins, thanks to his parents' efforts and his intelligence, he managed to become a teacher and move to Barcelona shortly before World War I.

[11] From 1926 onwards, he belonged to the "Left Opposition" led by Trotsky, which opposed the rise of Joseph Stalin within the Communist Party of the Soviet Union,[12] so Nin had to leave the USSR in 1930.

After earlier criticism of his political line, he ended up breaking with Trotsky after he did not accept his attempt to adopt an entryist tactic in the PSOE.

When his group merged with Joaquín Maurín's Workers and Peasants' Bloc (BOC) to found the Workers' Party of Marxist Unification (POUM) in 1935,[14] Nin was appointed a member of the new party's executive committee and editor of its publication, La Nueva Era; the following year he was elected general secretary of the POUM.

[16] In the autumn, Nin had raised with the President of the Generalitat, Lluís Companys, the possibility of taking in as a refugee Leon Trotsky, who at that time had been forced to leave Norway under Soviet pressure.

On 24 November, the PSUC handed the CNT a proposal on the establishment of a new government of the Generalitat, which included the departure of Nin as Minister of Justice.

[29] On 14 June the Director General of Security, Colonel Antonio Ortega Gutiérrez, informed the Minister of Education and Health that the head of the NKVD in Spain, Alexander Orlov, had indicated to him that all POUM leaders should be arrested.

The NKVD chief claimed that there was evidence linking the Trotskyist party to Franco's espionage and that it was necessary for the government not to be aware of this plan because the Minister of the Interior, Julián Zugazagoitia, was a friend of some of the POUM leaders.

[30] On 16 June the Republican authorities closed down the POUM headquarters in the Hotel Falcón and the party leadership was arrested by the police.

[31] Andreu Nin was separated from the rest of the party leadership, like Julián Gorkin and José Escuder, who were held in prisons in Madrid and Barcelona.

[31] Nin was transferred to the city of Alcalá de Henares, near Madrid; the chosen location had become an important Soviet base in Republican Spain and therefore offered guarantees of security.

Hugh Thomas notes that Nin was transported by car from Barcelona and then taken to the Cathedral of Alcalá de Henares, which functioned as a private prison of the Soviet NKVD.

According to Ricardo Miralles and Hugh Thomas, Negrín would have been aware of the truth about what had happened from the beginning despite echoing the Gestapo's implausible version;[43] Thomas adds that the Nin case was in fact a 'dirty affair', but that the Republican leaders decided it was better not to bother the Soviets in order to continue receiving the precious military aid.

[42] In February 1938 a hit-squad related to POUM and Sección Bolchevique-Leninista de España shot a Soviet agent held responsible for the detention of Nin, Leon Narwicz.