Naismith became a naturalized US citizen in 1917 in preparation for his service during World War I.

Naismith served with the YMCA in France during World War I. He worked to promote morale and provide recreational activities for American soldiers stationed overseas. He supported soldiers' physical and mental wellbeing during this period.

After the war, Naismith returned to his teaching duties at the University of Kansas. His experiences during the war further solidified his commitment to promoting health and well-being through physical activity.