Pere Mestres i Albet (1901-1975), professor of electrical expert, founder of ERC, Minister of Interior of the Government of Catalonia (October, 1933 - December, 1933) and Public Works and Health (1933-1936).
The party was founded in March 1931 as the union of the independentist Estat Català (Catalan State), led by Francesc Macià, the
Catalan Republican Party, led by Lluís Companys and the L'Opinió Group of Joan Lluhí i Vallescà. The party had done extremely well in the municipal elections of 12 April 1931. Two days later, on 14 April, few hours before the proclamation of the Spanish Republic in Madrid, Macià declared in Barcelona that Catalonia would become an independent republic within the Iberian Federation. This was not exactly what had been agreed in the Pact of San Sebastián, so three days later they negotiated with the Madrid government that Macià would become president of the Generalitat of Catalonia, an autonomous Catalan government inside the new Spanish Republic. In September 1932, the Spanish Republican Cortes approved the Statute of Autonomy of Catalonia which, among other provisions, granted a Catalan Parliament with full legislative powers, and it was elected on 20 November 1932. The Republican Left of Catalonia won a large majority of seats.
On 6 October 1934, Lluís Companys, elected by the Parliament of Catalonia as new President of the Generalitat after the death of Francesc Macià in December 1933, unlawfully declared a Catalan Republic within a Spanish Federation, following the entry of right-wing ministers of the Spanish Confederation of the Autonomous Right (CEDA) into the Government of the Spanish Republic. The party leaders (including Companys itself) and the Catalan Government were sentenced by the Spanish Republic and jailed, and the Statute of Autonomy of Catalonia was suspended until February 1936.
In 1936, at the dawn of the Spanish Civil War, ERC decided to become part of the Popular Front to contest that year's election, which it won. Esquerra became the leading force of the Popular Front in Catalonia and during the war tried to maintain the unity of the Front in the face of growing tensions between the Workers' Party of Marxist Unification (POUM) and the pro-soviet Unified Socialist Party of Catalonia (PSUC).
The party was declared illegal (along with all other participants in the Popular Front) by Francisco Franco after he came to power in 1939. The former president of the Catalan Generalitat, Lluís Companys, was arrested by German agents in collaboration with Vichy France, returned to Spain and executed on 15 October 1940.
The party is also federated with parties in the Balearic Islands and in Northern Catalonia in France, as well as with Republican Left of the Valencian Country in the . Except for their Balearic counterpart, none of the latter currently have any parliamentary representation in their respective territories, though they do have 8 municipal councillors in the Balearic Islands and 6 councillors in the Valencian Community.