[1][3] Willcock became an active and enthusiastic member of the Labor Party (ALP), holding various offices in the Geraldton branch of the Engine Drivers', Firemen's and Cleaners' Union, and representing them at Federal conferences.

On 14 April 1917 he contested the Western Australian Legislative Assembly seat of Geraldton in a by-election occasioned by the resignation of Edward Heitmann, but was defeated by Samuel Elliott.

When the Labor party returned to power under Collier on 24 April 1933, Willcock resumed as Minister for Railways and Justice, and was also given the Education portfolio.

On 19 August the following year, Collier stepped down as premier and leader of the Labor Party, and the following day Willcock was appointed his successor.

He dropped the Forests portfolio on 9 December 1943, but held the other offices for nearly nine years, resigning for reasons of ill health on 31 July 1945.

He died on 7 June 1956 at St John of God Hospital, Subiaco, Western Australia, and was buried in Karrakatta Cemetery.