John N. Hagan was born on August 4, 1873, near Arcola, Indiana. He was the son of William and Wilhelmina (Rapp) Hagan. At age nineteen, he enrolled at Valparaiso University (Indiana), and graduated with a Bachelor of Science degree in 1900. In the Fall of the same year, Hagan accepted the position of Superintendent of the public school at St. John, North Dakota. In 1903, Hagan resigned and moved to a homestead near Deering, North Dakota.
Hagan joined the newly formed Nonpartisan League in 1916, becoming one of the earliest members. At the first Nonpartisan League State Convention in 1917, Hagan was unanimously nominated for the office of Commissioner of Agriculture and Labor. In the November election, Hagan was successful in his bid for office. Elected to a second term in 1920, Hagan was ousted in the Recall Election of 1921. Six years later Hagan was appointed Deputy Administrator of the Prohibition Board until its abolishment in 1933.
In 1936, Hagan was again endorsed by the Nonpartisan League and elected Commissioner of Agriculture and Labor. In 1938, Hagan ran for governor but was defeated by John Moses, the Democratic contender. Following this defeat, Hagan retired from active politics and pursued farming until his death on June 4, 1952.
He married Rhea Smith on April 19, 1904. They had two children, Stephen and John, Jr.