McGirr was first elected to the New South Wales Legislative Assembly in 1922 and served continuously until 1952, holding the various seats of Cootamundra, Cumberland, Bankstown, and Liverpool.

Having served in the third ministry of Jack Lang, and the first and second ministries of William McKell, McGirr was variously torn between Lang Labor and the newly formed Australian Labor Party.

There was a rearrangement of the Ministry in September 1949, triggered by the resignation of Deputy Premier, Jack Baddeley.

Baddeley suffered a heart attack in December 1948 while serving as Acting Premier.

The portfolio of Co-operative Societies was created and filled by Clarrie Martin.