Between 1969 and 1973 Alfonsas Eidintas studied history at Vilnius Pedagogical University, going on to serve as chief lecturer, docent, head of the Universal History Department, and Deputy Dean at that institution. From 1986 to 1993 he was the Deputy Director for Research at the Lithuanian Academy of Sciences' Institute of History, receiving his habilitation in 1990.
Eidintas entered diplomatic service in 1993, serving as Lithuania's Ambassador to the United States. He has also served as Lithuania's Ambassador Extraordinary and Plenipotentiary to Canada, Mexico, Norway, Israel, Cyprus, Ethiopia, South Africa, and Nigeria. The Lithuanian diplomat also worked in the Ministry of Foreign Affairs, most recently as ambassador-at-large at the Foreign Ministry's Information and Public Relations Department; and was a lecturer at the Institute of International Relations and Political Science.
As of 2009, he had authored 17 books and at least 50 scientific articles about Lithuanian history and politics, and published a work of historic fiction, Ieškok Maskvos sfinkso (In Search of the Moscow Sphinx).