He was educated at Scotch College, Melbourne, where he excelled at mathematics and English. His first major success came when he secured an appointment as a cadet-mathematical master in the Public Works Department. His first book, The Citadel (1905), was based on his experiences at this time, when he also began to write fiction.
He presided over the Australian Literary Society in 1912 and 1913 and is credited with having started this society's policy of publishing new poets in the magazine Poetry Australia (first published in 1914). His second novel, The Bushranger (1906), based on his experiences in the goldfields (and also written in collaboration with Charles Harpur), is considered one of the most important novels of Australian literature. Cronin's subsequent novels became less successful than his earlier works; he developed so many interests that he could not find time to devote to writing fiction.
He wrote extensively on political, social, and economic issues and also produced fictional works of considerable power and realism. His four-volume biography of Joseph Stalin is considered one of the major biographies of this period. Cronin died in 1958 at the age of 86.