Haylen enlisted in the Australian Imperial Force on 6 July 1918 and was sent to Europe, but his ship was recalled and he was discharged in January of the following year.

His first play, the anti-war Two Minutes' Silence, was first staged in 1930; it received positive reviews and ran in Sydney for twenty-six weeks.

As a consequence, his contract with Consolidated Press Ltd was terminated by its owner, Frank Packer, and Haylen then became editor of the official newspaper of the ALP, the Standard.

He scored an unexpected victory in the 1943 federal election over the long-standing member Sir Charles Marr, attracting 52.9% of the two-party-preferred vote.

Haylen was overlooked for a cabinet post after the 1946 elections, but led a parliamentary delegation to Japan in 1948, where he sparked controversy by shaking hands with Emperor Hirohito.

Haylen's memoirs Twenty Years' Hard Labor were published in 1969, revealing his disillusionment with parliament and aspects of the ALP, especially its right wing.