Hagar, the third Principal of Salem Normal School, was born in Newton Lower Falls, Massachusetts on April 22, 1820. Hagar was graduated from Union College in 1843 as valedictorian of his class and a member of Phi Beta Kappa. Prior to his appointment to Salem Normal, Hagar was Principal of Canajoharie Academy, Superintendent of Schools in New York State, principal of Norwich Academy, and Headmaster of Eliot High School in Jamaica Plain. He also served as President of the Massachusetts Teachers Association and co-founded the National Teachers Association, which became the National Education Association while he was president of the organization.

Many changes occurred during Hagar's tenure as Principal of Salem Normal. The school expanded and renovated its building on Broad Street in 1871 because of increasing enrollment. The curriculum grew with more emphasis given to psychology, music, drawing, calisthenics, and industrial arts. Hagar also developed the idea of a training school for Salem Normal students but he would not live to see the completion of the new school building he planned, which would devote the entire first floor to a model school. Dr. Hagar died in Sharon, Massachusetts on July 4, 1896.