[4] The 1903 New South Wales referendum required the number of members of the Legislative Assembly to be reduced from 125 to 90,[5] and the Manning was one of the abolished seats.

[1][6][7] He stood again as an independent Nationalist candidate for the state seat of Oxley at the 1927 election, but was comfortably defeated.

[8][9] In 1906, Thomson was elected to the Australian House of Representatives as a Protectionist, defeating Henry Lee of the Anti-Socialist Party for the seat of Cowper.

He was defeated in 1920 by future Prime Minister of Australia Earle Page, who contested for the Farmers and Settlers Association.

He also purchased a grazing and dairying property at Mount George, and increasingly retired there apart from his board duties in his later years.