Gerő was born in Terbegec, Hungary (now Trebušovce, Slovakia) to Jewish parents, though he later totally repudiated religion. An early Hungarian communist, Gerő fled from Hungary to the Soviet Union after Béla Kun's brief communist government was overthrown. During his two decades living in the USSR, Gerő was an active KGB agent. Through that association, Gerő was involved in Comintern -- the international organization of communists -- in France, and also fought in the Spanish Civil War.
Erno Gero was a member of the High National Council between January 26, and May 11, 1945.
In the November 1945 elections, Hungary, the Hungarian Communist Party, under Gerő and Mátyás Rákosi got only 17% of the vote, compared to 57% for the Smallholders' Party. However, the Soviet Commander in Hungary, Marshal Kliment Voroshilov installed a coalition government with Communists in key posts. The Communists took control in 1947, with Rákosi as Prime Minister (and effective head of state). Rákosi's authority was undermined by Khrushchev's secret speech denouncing Stalinism, and he was forced to leave office on 18 July 1956, though he was able to designate fellow hardline Stalinist Gerő as his successor.
Gerő was forced by the Soviet Politburo to resign on October 25, 1956, during the second day of the Hungarian Uprising, after being blamed for ineptly responding to the uprising. After being replaced by János Kádár, Gerő fled to the Soviet Union. He remained in the USSR until 1961, when he returned to Budapest. Gerő worked in Budapest as an occasional translator until his death.