This thesis explores how the medicalization of negative emotions impacts political agency. Mental suffering apparently abounds in the modern world. Diagnoses of and medical prescriptions for mental disorder have for decades been surging among liberal democracies, including the UK. Critics argue that this is partly because negative emotions are increasingly being understood and treated as symptoms of mental disorder; that is, they are being medicalized. This has wide-ranging political implications. Negative emotions, such as anger, sadness, and fear, alert us to and drive us to address perceived wrongs. If we are able to relate our emotions to issues that we share with others, they can engender political action. But labelling an emotion as a mental disorder frames it as a medical problem, potentially foreclosing the possibility of comprehending the emotion as an understandable reaction to something that could and should be addressed politically. The medicalization of negative emotions, thus, appears to threaten our political agency as liberal-democratic citizens. To explore whether, and, if so, to what extent this is the case, I draw on the political thought of Hannah Arendt to construct a theoretical framework explicating the relationship between political agency, its sources, and negative emotions. Within this framework, political agency is defined as the individual’s capacity to act in concert with other people to shape or respond to public issues. I then apply this framework to a series of historical and contemporary case studies of political action by people whose emotional and mental fitness for public life has been called into question. I argue that the medicalization of negative emotions has generated factors that can be and have been used to undermine people’s political agency. However, I also contend that the disempowering effects of medicalization are more uneven and less severe than many critics have suggested.