The son of a Jewish village clerk, Kun became active in Social Democratic politics early in life, working at first in Transylvania and later in Budapest. He was mobilized in the Austro-Hungarian army at the outbreak of World War I, became a prisoner of war in Russia in 1916, and joined the Bolsheviks. Attracting the attention of V.I. Lenin, Kun received training in revolutionary tactics and returned to Hungary after the collapse of the Central Powers in November 1918. He started a communist newspaper and founded the Hungarian Communist Party on Dec. 20, 1918. Though imprisoned in February 1919 by the government of Mihály Károlyi (Károlyi, Mihály, Count), Kun was allowed to continue directing Hungary's Communist Party from his cell. His extensive propaganda combined social agitation with promises that, if given power, he would secure Soviet aid against the Romanian forces then occupying parts of Hungary.

On March 20, 1919, Kun was released by Károlyi, and the following day, as commissar for foreign affairs, he assumed the dominant position in a new Communist–Social Democratic coalition government. His regime took advantage of an upsurge of popular nationalism and created a Red Army that rapidly reconquered a considerable portion of the territory lost to Czechoslovaks and Romanians. Kun also quickly eliminated the moderate elements in the government through terroristic measures. Soviet help, however, failed to arrive, and Kun alienated the peasantry by nationalizing Hungary's estates rather than dividing them among the peasants. As a consequence, food distribution broke down, and the army refused to fight. The regime collapsed on Aug. 1, 1919, and Kun fled to Vienna. As a leader of the Third International, he attempted to initiate revolutionary outbreaks several times in Germany and Austria during the 1920s. He was eventually accused of “Trotskyism” and fell victim to one of Joseph Stalin's purges in the late 1930s.

Though possessing great energy and shrewdness, Kun was rigid in his communist views and was oblivious to the unpopularity of his policies during his brief rule in Hungary. Despite his organizational talents, he was unable to master the complexities of actual government or the tactics of power struggles within the international communist movement.