Warnecke began his journalism career in 1913 as a junior reporter for The Evening News.

In 1915, Warnecke enlisted in the Australian Imperial Force and served on the Western Front. Wounded in 1916, he launched and edited a small review entitled The Hurdcott Herald while convalescing in England.

After his discharge from the army, Warnecke continued at The Evening News and became active in the Australian Workers' Union. He become chief-of-staff at the new Daily Mail.

In 1923, he launched the London office of Smith's Weekly and its new Daily Guardian. He became chief sub-editor of the Daily Guardian in 1926, editor of the new Sunday Guardian in 1929, and later Editor-in-Chief of Frank Packer's Australian Consolidated Press responsible for relaunching The Daily Telegraph.

The Australian Women's Weekly was launched in May 1933 with Warnecke as its editor and regular contributor, along with other prominent journalists and writers. It was highly successful and remained the highest selling per capita magazine in the world for its first 50 years.

After Warnecke resigned from Consolidated Press in 1939, he studied printing and magazine methods in the United States for the Herald and Weekly Times company and wrote for the Melbourne Herald. In 1940 he became a foreign correspondent for the McClure Newspaper Syndicate and in 1943 joined the US Office of War Information as a special writer.

In 1947, Warnecke returned to Australia and co-founded Atlas Publications with his fellow journalists Jack Bellew and Clive Turnbull. Atlas published magazines and popular fiction, but is best known for its comics and the successful Captain Atom series. When the Australian comic industry declined and Jack Bellew died (in 1957), the company ceased publication in 1958.

That year, Warnecke relocated to Ireland with his Irish-born wife, who had an active career as a writer and singer.