His father was a prosperous farmer and a leading local member of the Croatian Peasant Party, who during World War II became a deputy on the State Anti-fascist Council for the National Liberation of Bosnia and Herzegovina or (abv.

He became a deputy for Bugojno and for the West Bosnian district, and in 1965 secretary of the Central Committee of the League of Communists of Bosnia and Herzegovina – before being elected its president in 1969.

[2][4][5] He insisted on distinctiveness of Bosnia and Herzegovina which was characterized by historical permeation of cultures and customs that "shaped the man of this land" and enforced a unique and distinctive "Bosnian soul", but he never missed to emphasize the value of "unity of particularity" and "particularity in unity", as well as the fact that Bosnia and Herzegovina is the "motherland of all those who live there".

[2][4] In 1970, prior to the 1971 population census in Yugoslavia, Mikulić confronted a group of older generation of Bosnian officials, including two powerful Muslim politicians, who complained to Tito that the (m)uslim national status does not need any further resolution nor de iure verification, because, as they contended, (m)uslims are merely an Islamised Serbs, Montenegrins, Croats, and Yugoslavs, after which Tito requested Mikulić's immediate presence and explanation.

[2] While working within the communist system, Mikulić joined forces with a group, who belonged to a second generation of post-WWII politicians, and which included Džemal Bijedić, Milanko Renovica and Hamdija Pozderac.

[2][4] They strived to reinforce and protect the sovereignty of Bosnia and Herzegovina and achieve full national equality on the basis of both ZAVNOBiH and Second AVNOJ conclusions, and with a system reform in 1971, which eventually resulted in new constitution of 1974, they were successful.

[2][4] Their efforts proved key during the turbulent period following Tito's death in 1980 and are today considered some of the early steps towards Bosnian independence.

In March 1987, Mikulić was publicly rebuked for his economic policy by striking workers who refused to serve him while in Kranjska Gora for a ski-jump competition.

[17] In the 1990s, in an interview for Nedeljna Dalmacija he stated, that in the event of the breakup of Yugoslavia, he would remain with those "who choose a sovereign and independent Bosnia and Herzegovina and its territorial integrity within the existing borders.

[17] He contributed to underdeveloped Bosnia and Herzegovina post-WWII industrial and economic development like no other politician and was main force behind its rise as equal among the Republics at the Federal table.