Born in Newry, educated at a Jesuit school in Dublin, John O'Hagan studied Law and Arts at Dublin University. There he became friendly with Thomas Davis, Gavan Duffy, and other Young Irelanders. He wrote for the Nation newspaper and was the author of some of its best known ballads. He toured Munster with Duffy and the poet Denis Florence McCarthy, and Ulster with Duffy and John Mitchel, and published accounts of both adventures, which cast light on the country side and people during the 1840s. After the 1848 revolution, O'Hagan worked as a lawyer on the Munster Circuit. Subsequently, he became friendly with John Henry Newman and lectured in Law, Literature and the Arts in Newman's Catholic University. He stayed in touch with Newman after the latter had returned to England.

In the 1860s, O'Hagan was appointed a Commissioner for National Education, a post and subject of great interest to him up to his death. In that decade also he married Frances O'Hagan, who was much younger than him. They had a happy marriage and their house on the hill of Howth was a welcome centre for poets such as Gerard Manley Hopkins and Aubrey de Vere, and a range of friends, writers, educationists, lawyers, and clergy. John O'Hagan prospered in a career in equity law, and he was appointed in turn chairman of the court of quarter sessions in Leitrim and in Clare. While in Clare, the title was raised to that of Judge. In 1880 he was appointed to take charge of the land commission arising from Mr Gladstone's Land Act of 1881. He died in 1890 widely mourned and praised as a man of integrity who, in the words of The Spectator magazine, was 'known to all not only as a most learned and experienced lawyer with a serene temper and a judgement of rare balance, but as a scholar of wide and liberal culture, a man beloved and respected by all who knew him'.