McCabe was born at 14 Chestergate, Macclesfield, Cheshire, but his family moved to Manchester, near Gorton Monastery, while he was a child. He trained there as a Franciscan Friar from the age of 15. His novitiate year took place in Killarney, after which he was moved to St Bonaventure's School, Forest Gate in London. He was ordained as a priest in 1890 and given the name "Father Antony". He studied philosophy at the Catholic University of Louvain for a year, and in 1895 was appointed rector of the newly founded St Bernardine's College, Buckingham, but he had gradually lost his faith and on Ash Wednesday, 19 February, 1896 he resigned and renounced the church. He wrote about this period of his life in the first of his many books Twelve Years in a Monastery (1897), and also in a novel In the Shade of the Cloister (1907) published under the name "Arnold Wright".
From 1896 he worked with Stanton Coit and F. J. Gould in the Union of Ethical Societies, founded that year in London, but in 1898 moved to Leicester as secretary of the Secular Society. It was there that he met Beatrice Lee, a hosiery worker, who became his wife. They were married 17 August 1899. He found he was not suited to the pastoral work required of him, and after a year returned to London to become the first director of the Rationalist Press Association, until 1902. Many of his earliest works were written for the RPA, including The Life and Letters of George Jacob Holyoake (1908).
For the rest of his long life he made a living as a freelancer, writing particularly on the history of the Catholic church. He was also a pioneer in the popularisation of science, with his translation of Ernst Haeckel's Riddle of the Universe (1900), and titles such as Evolution of Mind (1910), The Story of Evolution (1912) and The Evolution of Civilization (1922). McCabe was also in demand as a speaker, and gave 3000 - 4000 lectures in his lifetime, making speaking tours in North America and Australia, as well as great Britain.
In 1925 he and his wife separated, they had raised two sons and two daughters. About the same time he also made a break with the RPA, and from 1926 wrote many works for the American freethought publisher E. Haldeman-Julius in his "Blue Book" series. Many of these were published in part-work format, accumulating to form a larger work. For example The True Story of the Roman Catholic Church (1930-31) in six double volumes, adding up to 768 large pages. He wrote (1926): "I am what is called a Feminist. Thirty years ago I left a monastery and began a sane human existence. Within two or three years, I find, I was defending the rights of women." And in An Outline of Today's Knowledge (1932): "Civilization has really not yet begun, so prophecies of its doom are misplaced. We must not too easily despair. We are but the children of the dawn."
One of his last works was A Rationalist Encyclopaedia (1948). In it he wrote: "The Rationalist case needs no straining of evidence and always gains by the severest self-criticism." (p.114). It was McCabe who started the controversy over the pro-Catholic censorship of the later editions of Encyclopedia Britannica, which omit sections from the 11th edition that were critical of the church.
He seems to have made very different impressions on people. Marciet Haldeman described him as: "A spare, wiry fellow of medium height and brisk gait. ... Clear blue eyes behind spectacles ... Gray, thinning hair ... Clean-shaven face, ascetic, yet illuminated by a merry twinkle that somehow finds its way into his lively, very English, speech. ... A gentleman whose charm is reinforced by encyclopedic knowledge." [quoted by Goldberg] Whereas Nicholas Walter wrote: "He was always a devastating controversialist, and often a difficult colleague. He himself attributed his irascible and pugnacious temperament to lifelong dispepsia, and he had a grim face and voice to match. He had little respect for most of the leaders of the organisations he worked for." [Dictionary of National Biography]
McCabe died aged 87 at 22 St George's Road, Golders Green. The epitaph he requested was: "He was a rebel to his last day."