John Stuart Mill, whose relevance for Nietzsche’s late work has been documented by recent research, is not directly mentioned in On the Genealogy or Morality (1887). This article argues that Mill’s On Liberty (1859) nevertheless played a crucial role in the development of the Genealogy. Following a source-based methodology, three major references demonstrate how Nietzsche used On Liberty as a resource as he initiated and developed his own exploration into the origin of morality. After tracing Nietzsche’s reading of Mill through (...) pencil marks in his copy of On Liberty, I discuss the reappearance of Mill’s motives and terminology as well as the explanatory power of Mill as a source for the respective sections in the Genealogy. It becomes clear to what extent Nietzsche adapts Mill and how he turns Mill’s own weapons against him through skillful transformations. The article concludes that instead of delivering the “better” Mill, Nietzsche builds up a fruitful tension between a then-common take on morality and its transgression. (shrink)