Mousavi was the last Prime Minister in Iran before the constitutional changes which removed the post of prime minister in 1989.

Mousavi is well remembered by many Iranians for managing the country during the Iran-Iraq War (1980-1988) and for his very effective steering of the country out of an economic recession.

Mousavi has directly addressed activating foreign policy to boost national interest by reducing tensions with other nations. This includes negotiating with U.S. President Barack Obama if “his actions are in keeping with his words”. His other notable assertions were when he called Ahmadinejad’s approach to the issue of Holocaust a wrong one. Mousavi condemned the killing of Jews in the Holocaust, a much different stance than Ahmadinejad.

From here:
Biography: Mir-Hossein Mousavi

Other sources that show past was different then his position now:

Iran Tracker

He was anti Shah, anti-US:

Profile: Mir-Hossein Mousavi, Iran’s Radical Turned Reformist