Mohamed Siad Barre was born on October 6, 1919, near Shilavo, a town in the predominately Somali-populated Ogaden region of the Ethiopian Empire, into the Somali Marehan Darod clan and the sub-clan of Rer Dini. Barre's parents died when he was ten years old, and after receiving his primary education in the town of Luuq in southern Italian Somalia moved to the capital Mogadishu to pursue his secondary education. In 1935, Barre enrolled in the Italian colonial police as a Zaptié despite being ineligible as he was born in Ethiopia, instead claiming to have been born in Garbahaareey in order to qualify. Barre seems to have probably participated as a Zaptié in the southern theatre of the Italian conquest of Ethiopia in 1936, and later joined the colonial police force during the British Somaliland military administration, rising to major general, the highest possible rank. In 1946, Barre supported the Somali Conference (Italian: Conferenza Somala), a political group of parties and clan associations that were hostile to the Somali Youth League and were supported by the local Italian farmers. The group presented a petition to the "Four Powers" Investigation Commission in order to allow that the administration of the United Nations Trust Territory could be entrusted for thirty years to Italy. In 1950, shortly after Italian Somaliland became a United Nations Trust Territory under Italian administration for ten years, Barre (who was fluent in Italian) attended the Carabinieri police school in Florence for two years. Upon his return to Somalia, Barre remained with the military and eventually became Vice Commander of the Somali Army when the country gained its independence in 1960 as the Somali Republic.
In the early 1960s, after spending time with Soviet officers in joint training exercises, Barre became an advocate of Soviet-style Marxist-Leninist government, believing in a socialist government and a stronger sense of Somali nationalism.