Born in Auburn in the Clare Valley north of Adelaide, Dennis was educated at St Aloysius' College, Sevenhill, and Christian Brothers College in Adelaide. He contributed to school magazines throughout. After leaving school he had a number of jobs including as a junior law clerk, barman and on the staff of the Adelaide journal The Critic. Dennis also worked for a time in New South Wales. His first verse was published in the Laura Standard newspaper when he was 19 years old. In 1901 Dennis went back to work for The Critic and was editor from 1904 to 1905. In November 1903 one of Dennis' works appeared in the Sydney periodical The Bulletin, a journal with which he would have a lasting association.

In February 1906 Dennis and AE Martin established the weekly journal The Gadfly to which Dennis contributed much material, including more than 200 poems and essays, under various pseudonyms. From late 1907 Dennis lived in Melbourne and worked as a freelance journalist for various newspapers and magazines. He moved to Toolangi, about 70 kilometres north-east of Melbourne in 1908 and lived there for the rest of his life.

Dennis' first book of verse, Backblock ballads and other verses, was published in 1913. He found fame two years later with the publication of The songs of a sentimental bloke, written in Australian vernacular and illustrated by Hal Gye. The humorous love story appealed to a nation at war and it became incredibly popular with Australian troops overseas. The work was produced as a silent film in 1918, a stage play in 1922, a sound film in 1932, a ballet in 1952 and a musical in 1961, as well as numerous recordings and radio and television programmes. Dennis followed Sentimental bloke with the publication, in 1916, of The moods of Ginger Mick, which told the story of an Australian soldier on the battlefields of World War One. Other works include The Glugs of Gosh (1917), Doreen (1917), Digger Smith (1918), Jim of the Hills (1919) and A Book for Kids (1921). Dennis returned to journalism in 1922, contributing a daily column to the Melbourne Herald for the next 16 years. There is a memorial statue of CJ Dennis at Auburn, South Australia, and a CJ Dennis festival is held in the town each year. There is also a commemorative plaque at the Southern Cross Hotel, Melbourne, and the gardens of his home at Toolangi, known as the 'Singing Gardens' also serve as a memorial. The CJ Dennis Prize for Poetry is awarded each year during the Victorian Premier's Literary Awards to the best new work by a poet published in a book.

1904-1905: Editor of The Critic

14 February 1906: Dennis and AE Martin launched the weekly journal The Gadfly

1913: Dennis' first book of verse, Backblock ballads and other verses, published

October 1915: The songs of a sentimental bloke published

1916: The moods of Ginger Mick published

1918: Silent film of The songs of a sentimental bloke produced

1922-1938: Contributed daily column to the Melbourne newspaper The Herald

Hutton, Geoffrey. C. J. Dennis : the sentimental bloke : an appraisal after 100 years of his birth, Melbourne : Premier's Dept., 1976

McLaren, Ian F. Talking about C. J. Dennis, Melbourne : English Department, Monash University, 1982

Australian Dictionary of Biography online: Search for Clarence Dennis