Lemma grew up in Ethiopia and obtained his bachelors from Addis Ababa University and his DSc from Johns Hopkins University in 1964 for his research on leishmaniasis. In 1964, he discovered a cheap and natural treatment against bilharzia or schistosomiasis, a disease which affects 200 million people globally. The treatment was from the fruit of a common African plant, endod, which possesses molluscicidal activity.
Lemma returned to Ethiopia as faculty member at Addis Ababa University, and later founded the now named Aklilu Lemma Institute of Pathobiology. He initiated the founding of the Ethiopian Academy of Sciences, but this was impeded by the revolution and Derg Regime and would be picked up again in the early 2000s. In 1976, Lemma worked for the United Nations and became the Deputy Director of UNICEF's International Child Development Centre and later became a professor in the Department of International Health at Johns Hopkins. He was awarded the Right Livelihood Award of Sweden in 1989.