John Harvey Kellogg married Ella Eaton, a teacher and health reformer who shared his commitment to Seventh-day Adventist principles. They had a complex relationship, and it is believed to have been sexless.

John Harvey and Ella Kellogg did not have biological children, but they adopted numerous children over the years, providing them with a home and education.

The relationship between John Harvey and his brother Will Keith Kellogg was often strained due to their differing views on business and health. The corn flakes controversy created a deep rift between them that never fully healed.

Kellogg mentored and influenced many individuals who worked at the Sanitarium and shared his health principles. He was a charismatic leader but could also be demanding and controlling.

Even in his later years, Kellogg remained dedicated to his health principles and continued to promote them through his writings and lectures. He maintained a rigorous lifestyle and remained active until his death.