Ayatollah Ruhollah Khomeini effectively came to power in early 1979, during and immediately after the Iranian Revolution.

More specifically:

  • He returned from exile to Iran on 1 February 1979, becoming the de facto leader of the revolution.
  • The Shah’s regime collapsed and by 11 February 1979 the armed forces declared neutrality, marking the victory of the revolution and Khomeini’s practical control over the state.
  • A national referendum in March–April 1979 created the Islamic Republic, and Khomeini was established as Iran’s supreme political and religious leader for life under the new constitution later that year.

So, in short: Khomeini came to power with the success of the Iranian Revolution in February 1979, and was formally installed as supreme leader of the Islamic Republic later in 1979.