Webster was a member of Marrickville Municipal Council from 1887, and stood for the New South Wales Legislative Assembly seats of Canterbury, Petersham, and Wickham, only to be defeated in all of them. Although he had opposed Federation, he contested the seat of Gwydir in 1901 in an attempt to enter the Australian House of Representatives, but was unsuccessful. However, from 1901 to 1903 he was the first Labor member for Moree in the Legislative Assembly, and as a result of his success was elected to the seat of Gwydir in 1903. He was known for always answering letters and requests from his constituents personally.
His role in the royal commission on postal services was significant, and was in part responsible for the downfall of the Deakin government. On 9 July 1909, as part of these pursuits, Webster delivered a renowned speech lasting 10 hours and 57 minutes, a record which, under new time limits, can never be broken. As a result of this he became known as "the man with the iron jaw". He was Postmaster-General in the first Hughes government, and together with his Prime Minister he left the Labor Party in 1916 over conscription to join the new Nationalist Party of Australia. He retained his place in the government until the 1919 election, at which he was defeated.