Michael Joseph ("M.J.") MacManus was born in the town of Carrick on Shannon, in County Leitrim on February 10, 1888. His father, Patrick, was a schoolmaster and his mother, Maria, was the matron. M.J. was the eldest among his siblings, which included two brothers and three sisters. His early education was acquired at Marist Convent and the Presentation Brothers College, Carrick on Shannon. He then studied at Farnham College, Surrey and London University where he received his degree in teaching. He taught for a brief period in Wigan, Lancashire, but felt unsatisfied and so decided to return to London where he practiced freelance journalism in Fleet Street. In 1912 he married Bridget Glynn of Galway. They had one daughter, Molly, who married Robert MacNabb and lived in Carrick on Shannon until her early death in 1965. In 1916 MacManus returned to Ireland, where he contributed regularly to several newspapers and periodicals. His wife died in 1917, and he mourned her for many years.

M.J. MacManus was also an ardent nationalist. He befriended many National leaders, particularly Eamon De Valera and Michael Collins. He was arrested after the burning of the Customs House in Dublin.

In 1931 the Irish Press newspaper was begun, and MacManus was appointed Literary Editor, a position which he claimed until his death. He was particularly well known for his daily column entitled, "This Happened Today", in which he would highlight notable persons or events who had played an important part in Irish history.

In addition to his work as a journalist, MacManus published several books of parody includingA Green Jackdaw, A Jackdaw in DublinandDublin Diversions. His serious poetry includes two collections entitled Connaught Songs and Rackrent Hall. He also wrote a biography of Eamon De Valera, and made many radio broadcasts.

MacManus's main hobby was book collecting. At the time of his death he had over 6,000 books in his personal library, many of them rare first editions. M.J. MacManus died suddenly in September of 1951 while on holiday in Port na Blagh, Co. Donegal. He was buried in Glasnevin Cemetery.