Several events had taken place since the previous state election on 2 October 1937, which had changed the breakdown of the parties in the Assembly:
In November 1937, Ian Macfarlan (the member for Brighton) left the United Australia Party (UAP) and sat as an Independent. Macfarlan gave two reasons for his resignation from the UAP: that the party was sitting in opposition to the Dunstan Country government which he praised; and that it was controlled by a central body outside of the parliament, the members of which were unknown to most party members.Frederick Holden (Grant) also left the UAP and sat on the cross benches. This reduced the UAP's numbers from 21 to 19.
In August 1939, Alfred Kirton (Mornington) resigned from the UAP and sat briefly as an independent, before joining the United Country Party on 29 August 1939. This reduced the UAP from 19 seats to 18, and increased the UCP seats from 21 to 22.