He had quickly risen through the ranks of the communist party, and was quick to take advantage of any opportunity or mentorship. Allied with the KGB there was little challenge to his power. He was passed over the first time he was eligible for general secretary, mostly due to his younger age. But the second time, there was no one who could challenge him.
Once in power he domestic reforms and changed foreign policy. He began moving the country away from socialism to a more democratic movement, without ever stating he was doing so. His policies toward the world were ones of friendliness and reducing nuclear armament.
However his reforms were too late. The USSR was already beyond saving. In December 1991, faced with a broken economy, satellites breaking free, and Soviet states seeking independence Gorbachev resigned, and the Soviet Union was no more.