[2] In 1996, he defended his dissertation Maakaitseväekohustus Balti kubermangudes 19. sajandi 1. poolel (1806–1856) (Militia Duty in the Baltic Provinces in the First Half of the Nineteenth Century, 1806–1856), supervised by Helmut Piirimäe, in the History Department of the Faculty of Philosophy at the University of Tartu.

From 1999 to 2011, he was an adviser to the state archivist at the National Archives of Estonia, and from 2000 to 2013 he was an associate professor at the University of Tartu's Institute of History and Archeology [et].

Since 2012, he has been the research director of the National Archives, and since 2013 a professor at the Institute of History and Archeology.

Tannberg's main areas of research are the political and recent history of the Soviet Union, especially the Estonian SSR.

He has also been a member of the editorial board of the historical culture magazine Tuna [et] and a board member of the non-profit organization S-Keskus [et] since 1998, and since 1998 he has also been the editor-in-chief of the serial publication Uurimusi ja allikmaterjale Eesti sõjaajaloost (Research and Source Materials from Estonian Military History).