During the 1962 African Nationals Cup, Mengistu scored one goal in the final of the 3rd African cup against Egypt, when Ethiopia won their only major trophy to date. Ethiopia finished as top scorer of that tournament with four goals on the final day. He debuted with Saint-George SA in 1957 and remained with the club for the entirety of his career. Mengistu was given numerous, offers to play professionally for teams in Italy's Serie A and France, as well as Egypt's El Zamalek, but like earlier legend and coach Ydnekatchew, he refused all offers and stayed in Ethiopia wearing Saint George's characteristic "V" across his chest. Mengistu wore the number 8 for the entirety of his club and national team career. His international career began in 1958 and ended in 1970, following disappointment in the 7th African Nations cup in Sudan, where Ethiopia finished bottom of their group. He still managed to score three goals, the only Ethiopian goals in that tournament. Mengistu played two more years with Saint George, retiring in 1972. He is the seventh-highest scorer in the history of the African Cup of Nations with ten goals.