Chapman Cohen was the elder son of Enoch Cohen, a Jewish confectioner, and his wife, Deborah (née Barnett). He attended a local elementary school but was otherwise self-educated. He had read Spinoza, Locke, Hume, Berkeley, and Plato by the time he was eighteen.
Cohen recalled that he had "little religion at home and none at school", as he was withdrawn from Religious Instruction classes.
Cohen and his wife had two children; a son (who entered the medical profession) and a daughter, who died at the age of 29.
On his death, The Times printed a short obituary of Cohen, which said:
Secularist activism 
Cohen moved to London in 1889, and soon became involved in the secularist movement. Cohen commented that,
My introduction to the platform of the National Secular Society was quite accidental. I had heard none of its speakers, read none of its publications, except an occasional glance at Bradlaugh's National Reformer. I knew there was a Freethought movement afoot, but that was about all.
Cohen (1940, p. 61) relates that in the Summer of 1889 he was walking in Victoria Park when he came across a crowd listening to a Christian speaker:
the speaker was opposed by an old gentleman – at least he seemed old to me – who suffered from an impediment in his speech. The lecturer in replying spent part of his time in mimicking the old gentleman's speech. After he had "replied," the lecturer asked for more opposition. Mainly because of his treatment of the old man I accepted the invitation.
He spoke against the same lecturer – at their invitation – a few weeks later. Shortly afterwards he was invited to speak the local branch of the National Secular Society. After a year of lecturing for the freethought cause, he joined the NSS.
He was a popular lecturer for the Society, at his peak delivering over 200 lectures a year. He was elected a vice-president of the NSS in 1895.
In 1897 Cohen began contributing weekly articles to G. W. Foote's Freethinker, having previously written accounts of his lecture tours. In 1898 he became assistant editor of The Freethinker, and after Foote's death in 1915 he was appointed editor. Cohen had written for other freethought journals before joining The Freethinker, and had edited The Truthseeker, owned by J.W. Gott. Cohen also succeeded Foote as President of the National Secular Society.
According to Edward Royle (2004), "as an organizer Cohen did much to build up the resources of secularism in the inter-war years, but by 1949, when he was persuaded to resign as president, many members felt he had stayed on too long."
In 1940, summarising his own contribution to the Secularist movement, Cohen wrote:
For about forty-four years I have been busy in the interests of Freethought with my pen as well as with my tongue, and for about forty-two years I have been a regular writer for one of the oldest Freethought journals in Europe, and with a single exception, the oldest in the world. For twenty-four years I have been the official editor of that journal, and for the same period, President of the National Secular Society, the only organization for the propagation of militant Freethought in the British Isles. My career as a lecturer – continuously lecturing – is a record in the history of the Freethought movement.
Cohen remained editor of The Freethinker until 1951, when he retired and was replaced by F.A. Ridley.
Cohen criticised what he saw as mystical tendencies in the writings of some physicists, such as Arthur Eddington. Matthew Stanley interprets Cohen's materialism in Marxist terms, describing him as a "highly visible contemporary spokesman for socialist materialism", but noting that "It is unclear whether Cohen was a Marxist in a formal sense: he was a materialist, determinist socialist but he apparently never explicitly used dialectical reasoning or similar ideological resources... It seems likely that his anti-authoritarian attitudes prevented him from declaring formal allegiance to any system of thought, including Marxism." 
- Royle (2004). Said Cohen, "in common fairness I object to being called "self-education," as though I did everything myself." (1940, p.93).
- "I am afraid that if I said how many authors I read and the number of books I read, between the ages of thirteen and eighteen many would think I was romancing. But I can definitely date the fact that before I was eighteen I was familiar with Spinoza, Locke, Hume, and Berkeley, besides having revelled in the Platonic dialogues..." (Cohen, 1940, p.43)
- Cohen (1940, p.93).
- Cohen (1940, p.12).
- The Times, 6 February 1954, p.8.
- Cohen (1940, p.60)
- Cohen (1940, p.63).
- Cohen (1940, p.66).
- Cohen (1940, pp.7–8)
- Stanley (2008, p.189).
Works by Cohen 
- Almost an autobiography: confessions of a freethinker. London: Pioneer Press, 1940.
- Essays in freethinking: first series. London: Pioneer Press, 1923.
- Essays in freethinking: second series. London: Pioneer Press, 1927.
- Essays in freethinking: third series. London: Pioneer Press, 1928.
- Essays in freethinking: fourth series. London: Pioneer Press, 1938.
- Essays in freethinking: fifth series. London: Pioneer Press, 1939.
- Essays in freethinking: volume one. Reprint of Essays in freethinking, first and second series. Revised edition. Austin, Texas: American Atheist Press, 1987.
- Essays in freethinking: volume two. Reprint of Essays in freethinking, third and fourth series. Revised edition. Austin, Texas: American Atheist Press, 1987.
- God and the universe: Eddington, Jeans, Huxley and Einstein. London: Pioneer Press, 1931.
- A grammar of freethought. London: Pioneer Press, 1921.
- Materialism restated. London: Pioneer Press, 1927 (3rd edition 1943).
- Opinions, random reflections and wayside sayings. London: Pioneer Press, 1931.
- Religion and sex: studies in the pathology of religious development. London/Edinburgh: TN Foulis, 1919. Reprint, New York: AMS Press, 1975.
- Theism or atheism: the great alternative. London: Pioneer Press, 1921.
- The Other Side of Death: A Critical Examination of the Belief in a Future Life, with a Study of Spiritualism. London: Pioneer Press, 1922.
- War, civilization and the churches. London: Pioneer Press, 1930.
- Determinism Or Free Will?, 1919.
- Cohen, Chapman (1940). Almost an autobiography: the confessions of a freethinker. London: Pioneer Press.
- Royle, Edward (2004). "Cohen, Chapman (1868–1954)", Oxford Dictionary of National Biography, Oxford University Press, accessed 23 July 2009.
- Stanley, Matthew (2008). "Mysticism and Marxism: A.S. Eddington, Chapman Cohen, and political engagement through science popularization." Minerva, Vol. 46 (2), June, pp. 181–194. Online