Popular personalities like social media influencers (SMIs) and traditional celebrities can not only be used in the context of for‐profit marketing, but also help non‐profit campaigns address young audiences. Across three experiments (NTotal = 1233), we compared the effectiveness of SMIs’ mobilizing appeals in relation to a humanitarian issue to those made by traditional celebrities. Inspired by meaning transfer theory, we expected SMIs and their endorsements to be perceived as particularly authentic, elucidating their possibly unique influence on young people's participation. In Study 1, mobilizing appeals from popular personalities generally increased participation intentions among young people, but SMIs were not more effective in this role than celebrities. Study 2 and Study 3 showed that effects of the two source types are dependent on gender. Controlling for source and endorsement authenticity, we found that SMIs were better at directly mobilizing young women for political participation than celebrities. In contrast, young men were more easily convinced by celebrities to engage in political action, due to higher authenticity attributed to celebrities’ endorsements. While SMIs were not consistently perceived as more authentic than celebrities, the success of popular personalities’ mobilizing appeals can generally be well explained by perceived source authenticity and perceived authenticity of endorsements.