Volunteer service opportunities for older adults may soon be expanded. Although volunteering is thought to provide health benefits for healthier older adults, it is not known whether older adults in less than very good health are suitable candidates for high-intensity volunteering and can derive health benefits. This manuscript presents a prospective analysis of 174 older adult volunteers serving in Experience Corps Baltimore®, a high-intensity senior volunteer program in Baltimore, Maryland. Volunteers served ≥15 h per week, for a full school year, in elementary schools helping children with reading and other skills between 1999 and 2002. Volunteers were assessed with standardized questionnaires and performance-based testing including grip strength, walking speed, chair stand speed, and stair-climbing speed prior to school volunteering and at the end of the school year. Results were stratified by health status. Among 174 volunteers, 55% initially reported “good” and 12% “fair” or “poor” health status. At baseline, those in fair health reported higher frequencies of disease and disability than volunteers in excellent or very good health. After volunteering, a majority of volunteers in every baseline health status category described increased strength and energy. Those in fair health were significantly more likely to display improved stair-climbing speed than those in good or excellent/very good health (100.0% vs. 53.4% vs. 37.5%, p = 0.05), and many showed clinically significant increases in walking speed of >0.5 m/s. Satisfaction and retention rates were high for all health status groups. Clinicians should consider whether their patients in fair or good health, as well as those in better health, might benefit from high-intensity volunteer programs. Productive activity such as volunteering may be an effective community-based approach to health promotion for older adults.