He attended public schools in Newton until the age of thirteen, when he went to work in a paper factory.

At the age of sixteen, Hagar began an apprenticeship in a dry goods store in Boston.

[2] While at Canajoharie Academy, he worked with Susan B. Anthony, who served as headmistress of the female department.

When Hagar was working in Salem, Massachusetts, he attended one of Anthony's talks on women's suffrage and praised her argument.

[4] In 1857, Hagar attended the first meeting of what would become the National Education Association and wrote the organization's original constitution.