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Aregay was born in Adwa in 1927 by the Ethiopian calendar. He earned his BA in 1956 from what was called University College of Addis Ababa, now Addis Ababa University. From there, he was sent to earn an MA in education from Harvard University (1957), then an MA in history from the University of Chicago (1959). He completed his doctorate at the School of Oriental and African Studies in London (1971). He learned a variety of languages, both Ethiopian and foreign: "beside Amharic (Tigriñña, Gə’əz, some Oromo) and several European languages beside English (Italian, French, Portuguese)".  With his knowledge of Portuguese, he was the pre-eminent scholar on the history of the Portuguese Catholic influence and interaction in Ethiopian history.
His writings covered a variety of topics, regions, and periods of Ethiopian history. He is also remembered for his positive interactions with his many students, as he spent so much time in his office that it was his “second home”.