Mesfin Hagos was born in Azien, Eritrea. He was a founding member of the Eritrean People's Liberation Front (EPLF) in 1977. Before founding that organization he had joined the Eritrean Liberation Front (ELF) in 1966 as an ordinary member. He eventually would become Deputy Commander of ELF Zone 5.
Mesfin Hagos left the ELF in 1970 with Isaias Afewerki, Major General Asmerom Gerezgiher,Solomon Weldemariam and Tewelde Eyob. Hagos was a part of the founding leadership of the EPLF. Mesfin Hagos briefly served as the Chief of Staff in the EPLF. He was one of the commanders of the battle of Af'abet that destroyed the back bone of the Ethiopian army. In the middle of that battle, he was ordered by Isaias Afwerki to retreat to Nakfa but he turned off the radio message and liberated the town of Afabet.