Background: This study investigated how perceived knowledge, threat, and coping appraisals, moral obligation, and social approval influence voluntary mask-wearing intentions within an integrated framework combining Protection Motivation Theory (PMT) and the Theory of Reasoned Action (TRA). Understanding the drivers of protective behaviors after mandates are lifted is essential for sustaining public health practices. In the United States—where mask wearing is not culturally normative—such behaviors rely on internal motivations rather than external enforcement. Methods: Using data from a nationwide online survey of 360 U.S. adults, structural equation modeling was employed to test relationships among perceived knowledge, threat appraisal, coping appraisal, moral obligation, social approval, and behavioral intention. Results: Perceived knowledge significantly predicted both coping and threat appraisals. Coping appraisal influenced moral obligation, while threat appraisal affected both moral obligation and social approval. Among all predictors, only moral obligation significantly predicted behavioral intention, which in turn predicted protective behavior. Conclusions These findings highlight the central role of moral obligation and coping appraisal as key drivers of voluntary health protective intentions. The integrated PMT–TRA model provides actionable insights for sustaining motivation in low-threat contexts and offers guidance for shaping future public health strategies.