Born to middle-class parents who themselves had been involved in the Russian revolutionary movement of the 1870s, Kamenev became a professional revolutionary, joining the Russian Social-Democratic Workers' Party (1901) and its Bolshevik faction (1903). In 1908 he emigrated to western Europe, where he worked closely with the Bolshevik leader Vladimir I. Lenin and met his future wife, Olga Bronshtein, the sister of Leon Trotsky. In 1914 Lenin sent him back to Russia, but when Kamenev directed the Bolshevik delegates to the Duma (Russian parliament) to oppose Russia's participation in World War I, he was arrested and exiled to Siberia (November 1914).

After the February Revolution (1917), he returned to Petrograd (now St. Petersburg), assumed leadership of the Bolshevik organization there (with Joseph Stalin), and advocated conditional support of the provisional government. Although his policy was overruled by Lenin upon his return to Russia (April 1917), Kamenev retained his cautious approach to the revolution, opposing, with his close friend and colleague Grigory Zinovyev (Zinovyev, Grigory Yevseyevich), the Bolshevik decision to seize power. Despite his dissident views, he was elected to the Bolsheviks' first Politburo (October 1917), and, after the insurrection, he served as the first chairman of the Central Executive Committee of the All Russian Congress of Soviets. In 1919 he became a member of the reestablished Politburo and chairman of the Moscow Soviet (i.e., Moscow party boss).

When Lenin became seriously ill (1922), Kamenev joined Stalin and Zinovyev to form the ruling triumvirate, which politically attacked the commissar for war, Leon Trotsky (Trotsky, Leon), thus eliminating him from contention for power. Stalin then shifted his attack to Kamenev and Zinovyev (1925). Stalin succeeded in demoting Kamenev's status in the Politburo and removing him as head of the Moscow party organization. Kamenev later held other posts, but, after he, Zinovyev, and Trotsky formed a united opposition against Stalin (1926), he was removed entirely from the Politburo (October 1926) and from the party's Central Committee (November 1927) and was expelled from the party three times.

After the party leader Sergey Kirov (Kirov, Sergey Mironovich) was assassinated on Dec. 1, 1934, Kamenev was secretly tried and sentenced, with Zinovyev, for having indirectly contributed to the crime. In August 1936, however, he and Zinovyev were tried again in the first public-show trial of the Great Purge (purge trials). Accused of conspiring to assassinate Stalin and other Soviet leaders, Kamenev confessed to the fabricated charges in the vain hope of saving his family. He was shot, and his wife perished in the Gulag. Both Kamenev and Zinovyev were cleared of charges by the Soviet Supreme Court in 1988.