The Prime Minister led the Council of Ministers under both the Star Imperium and the United Planets.  Under the Imperium, the Imperial Sovereign designated one of the two Imperial Viceroys as Prime Minister and head of the Council of Ministers. When the office of Imperial Sovereign was vacant or the Sovereign was physically or mentally incapacitated, the Council of Twelve designated which Viceroy would serve as Prime Minister.

Under the United Planets, the United Planets Assembly elected and voted out the Prime Minister.