Mavromichalis was born into a historic family from Mani, Laconia that has contributed much to the Greek War of Independence of 1821. It is said that 69 members of the Mavromichalis family have died in battle from the time of the Orlov Revolt of 1770 to the Second Balkan War of 1913.
Stylianos Mavromichalis studied law in Athens, Zurich and Munich and arose to become President of the Greek Supreme Court (Areios Pagos).
In 1963, King Paul of Greece asked him to form a caretaker government that would lead the country into the November 1963 elections. Mavromichalis brief tenure as Prime Minister was exemplary assuring that fair elections would be held. In those elections, Georgios Papandreou of the Centre Union party was elected PM ending eight years of rule by the National Radical Union.
In 1965, when Papandreou was dismissed by King Constantine II, Stylianos Mavromichalis, in no uncertain terms, urged the young monarch to call for new elections as soon as possible. His call went unheeded and the 1965 crisis led eventually to the coup d' etat of April 21, 1967.