Thomson completed his education in England and trained at his uncle's business in Liverpool.

After spending two years at sea, he returned to Melbourne at the age of 19 and joined the merchant firm of Robert Harper.

[1][2] Thomson won the New South Wales Legislative Assembly seat of Warringah in 1894 and supported the Free Trade ministry of George Reid, although he opposed its legislation on workplace conditions, made necessary by its dependence on Labor Party support.

[1] In early 1904, following the retirement of William McMillan, he was elected deputy leader of the Free Traders.

[1] In July 1909, Thomson was elected president of the newly formed New South Wales Federal Liberal League.