He was active in church work, was Queensland Grand Secretary of the International Organisation of Good Templars and was also a trade unionist, serving as president of the Hairdressers' Employees Federation.

[3][4] In 1910, Finlayson was elected to the Australian House of Representatives as the Labor member for Brisbane, defeating Liberal minister Justin Foxton.

[10][11] An attempt to win Labor preselection for his old federal seat in 1925 was ruled ineligible due to his ties to the Victorian Anti-Liquor League, which had endorsed a non-Labor candidate for the Melbourne City Council.

[12] Finlayson relocated to Victoria during the 1920s, making an unsuccessful Labor candidacy for the conservative Victorian Legislative Assembly seat of Brighton at a 1928 by-election and serving as a City of Heidelberg councillor during the 1930s.

[13][14] He was an influential member of the Victorian General Assembly of the Presbyterian Church of Australia and was the convenor of its temperance committee.