Mass vaccination is considered necessary to reduce the spread of COVID-19; however, vaccination willingness was found to be especially low among young adults. Therefore, based on the extended Common Sense Model, the unique effects and the interplay of illness representations about COVID-19 and perceptions about COVID-19 vaccination in explaining COVID-19 vaccination willingness was investigated using a cross-sectional design. An online survey measuring the relevant variables was filled in by 584 participants (69.9% female) between 18 and 34 years. Correlation analyses showed that all illness representation dimensions except from timeline and both dimensions of vaccination perceptions were related to vaccination willingness. The mediation analysis revealed that less personal control, more prevention control, more concerns about COVID-19 as well as more perceived necessity of and fewer concerns about the vaccination were directly related to higher vaccination willingness. Additionally, prevention control was indirectly related to higher vaccination willingness through stronger perceptions of necessity of the vaccination. The extended Common Sense Model proved to be useful in the context of illness prevention. Campaigns to improve vaccination rates should aim at increasing the perception that COVID-19 is preventable through vaccination and the personal need of the vaccination as well as at decreasing concerns about the vaccination.