The present study examines Thucydides’ assessments of Pericles (2.65) and Alcibiades (6.15) drawing on advances from Leadership Studies. Moving away from conceptions of leadership as a quality of individuals, modern leadership theory views leadership as a relational process between leaders and followers. Thucydides’ assessments of Pericles and Alcibiades examine not only their effectiveness (i.e., their success or failure in conducting the war), but more importantly, the impact of their personal ethics on their relationship with followers. For Thucydides, both leaders displayed administrative competence, but their diverse adherence to ethical principles had a grave impact on their interaction with followers and consequently on their position as leaders. The comparative study of the two passages highlights how Thucydides’ understanding of leadership as a relational process anticipates an important strand of modern leadership theory according to which both effectiveness and ethics are inextricably intertwined in the concept of good leadership.