The present disclosure relates to an aerosol delivery device filling system. The system includes multiple source containers each respectively including a differing aerosol precursor composition. The system further includes a mixing container configured to engage the source containers to receive and mix the aerosol precursor compositions to form a mixed aerosol precursor composition. An aerosol delivery device may engage the mixing container to receive at least a portion of the mixed aerosol precursor composition. A related method for customizing an aerosol precursor composition is also provided.