Background/Objectives: Volunteering has been widely recognized as beneficial to the well-being of older adults, providing health benefits, increased social engagement, and a sense of purpose. This study aimed to explore the associations between volunteering and physical and mental health measures among older adults in Japan. Methods: Using an online survey conducted during the COVID-19 pandemic, 500 participants aged 65 and older were divided into volunteer and non-volunteer groups. Demographic, lifestyle, and health characteristics were comprehensively assessed using a series of reliable and validated instruments. Logistic regression analysis was used to explore the associations between volunteering and health outcomes. Results: The findings revealed that while there were no significant differences in physical health indicators and undertreatment of most diseases (with the exception of cataracts), volunteers reported significantly lower levels of depression and loneliness compared to non-volunteers. The volunteer group also showed greater engagement in social activities and hobbies, which may have contributed to their improved mental health outcomes. Conclusions: The results of this study add to the growing body of evidence suggesting that volunteering may be an effective, low-cost intervention for promoting mental health and social engagement among older adults.