Born in County Cork in 1924, Marcus was the much-loved editor of numerous anthologies of Irish fiction and poetry, including the Phoenix Irish Short Stories collections.

Other notable projects included the page New Irish Writing for the Irish Press, which provided a forum for aspiring Irish authors, publishing most of the most important names in Irish fiction, many for the first time, including Dermot Bolger, Ita Daly, Anne Enright, Neil Jordan, Claire Keegan, John McGahern, Bernard MacLaverty, Eilis Ni Dhuibhne, Joseph O'Connor, Colm Tóibín and William Wall. He was, in the words of Fintan O'Toole "the single most important literary editor in Ireland in the second half of the 20th century".His 1986 the novel A Land Not Theirs, a fictionalized account of the experiences of the Cork Jewish community during the Irish War of Independence was a bestseller. In 1986 his second novel which drew on his experiences among the Cork Jewish community, A Land in Flames was also a popular success.

In 2001 Marcus published Oughtobiography - Leaves from the diary of a hyphenated Jew, an autobiographical review of his life as an Irish-Jewish person and as a figure in the field of Irish literature.

On June 3, 2005, he was awarded an honorary Degree of Doctor of Literature by the National University of Ireland, University College, Cork

Marcus was married to fellow Irish novelist Ita Daly and lived in Rathgar in Dublin.