Nikita Khrushchev held the position until his removal from office in 1964. Nevertheless, he continued to exercise considerable control over the Politburo until his health began to fail seven years later. (PROSE: Who Killed Kennedy)
Konstantin Chernenko served as the General Secretary until his death in 1985. He was succeeded by the considerably more moderate Mikhail Gorbachev, whose tenure saw the fall of the Berlin Wall on 9 November 1989 and the collapse of the Soviet Union in the early 1990s.
In an alternative timeline created by the Elder Gods in the hope of destroying Earth, Chernenko was succeeded by the militant hardliner Vladimir Kryuchkov. This ultimately led to the outbreak of World War III on 9 November 1989 between the Soviet Union on the one hand and the United States and its allies including the United Kingdom on the other hand. Given that nuclear weapons were used by both sides, hundreds of millions of people were killed in the conflict. This timeline was ultimately negated by the Seventh Doctor. (AUDIO: Protect and Survive)