Gaining a public reputation for financial expertise, he was active in the Federationist cause and was a delegate to the 1897 Constitutional Convention, where he was a significant figure in the development of Commonwealth finance schemes.

Attempts to guide financial reform through the parliament as a backbench senator led to frustration, and Walker retired due to ill health in 1913 with his legislation unpassed.

There he supported equal representation for all states in an upper house, Commonwealth control of railways, and a national capital on federal territory.

[1] He reiterated his financial proposals, but was overlooked for the finance and trade committee, having been described by Alfred Deakin as "a mere commercial man".

[1] He also supported a minimum age for senators of twenty-five, rather than twenty-one, and voted for Paddy Glynn's proposal to include recognition of a "Divine Sovereign" in the Constitution.

[3] As a senator, Walker devoted his support to a variety of causes, including a transcontinental railway and equal pay for women.

[1] He also expressed reservations about the deportation of Kanaka labourers, supporting the reduction in their use but advocating that those resident in Australia for at least five years be permitted to remain.

[1] Re-elected as an Anti-Socialist in 1906, Walker expressed the view that the three-party system of the Australian Parliament's first decade was dysfunctional, and implored electors to vote "for either socialism or anti-socialism, and so end this triangular government".

He complained of the difficulty of passing private senators' bills, and at the 1913 election he retired, suffering from ill health.

He had been president of the Australian Golf Club (1903–19), a director of the Royal Prince Alfred Hospital, and a member of the Presbyterian Church's finance committee.