Early involvement

Through her volunteer work, Robertson became acquainted with social campaigner Bessie Rischbieth and state MP Florence Cardell-Oliver, who encouraged her to seek public office.^[3] She stood for the Nationalist Party in North Perth at the 1943 state election, but polled only 17% of the vote. In 1945, Robertson became a foundation member of the new Liberal Party. She served on the party's federal women's committee, as well as heading the local women's section and sitting on the state executive.^[1]

Senator for Western Australia

At the age of 67, Robertson became the Liberal Party's lead Senate candidate in Western Australia at the 1949 federal election, the first woman to head a major party's ticket.^[4] She was one of ten female candidates, but only she and Ivy Wedgwood of Victoria were successful. Their election brought the total number of women in the Senate to four (out of 60), the others being Dorothy Tangney and Annabelle Rankin.^[5] Tangney – 25 years Robertson's junior – was also from Western Australia, which became the first state to have multiple female senators.^[6] Robertson's first term began on 22 February 1950. She was re-elected to a second term at the 1951 election (a double dissolution), again running as the Liberal Party's lead candidate.^[1]

While attending a conference in Manila in early 1955, Robertson was informed via telegram that she had been dropped from her party's ticket for the next election. Her replacement was George Branson, a 37-year-old farmer.^[1] When pressed on the issue, Prime Minister Robert Menzies confirmed that Robertson's age was the primary factor in her exclusion – she would be 73 at the time of the election, which was thought to be too old.^{[lower-alpha 1]} After unsuccessfully attempting to have her non-selection overturned on procedural grounds, Robertson resigned from the Liberal Party on 7 September 1955. She initially sat as an independent, promising to continue supporting the government in the Senate to maintain its narrow majority,^[8] but on 28 September joined the Country Party – the first woman to represent the party in federal parliament.^[9] She was subsequently chosen to head its two-member Senate ticket.^{[lower-alpha 2]} Robertson mounted a vigorous 40-day campaign targeted at women voters, making a tour of rural areas and giving a series of radio broadcasts. She ultimately defeated Branson for the final Senate seat by about 17,000 votes, aided by Labor and DLP preferences.^[7] The Sydney Morning Herald described her re-election as "the most piquant situation produced by the election".^[1]

In the Senate, Robertson almost always supported the Menzies Government, and remained a personal admirer of Menzies even after his public lack of support for her candidacy. She did cross the floor on at least one occasion, in 1954 opposing a bill that required public servants to retire at the age of 65.^[1] Robertson took a keen interest in foreign affairs, and in 1956 became the first woman to serve on the Joint Committee on Foreign Affairs.^[10] She was firmly anti-communist, speaking in favour of the Communist Party Dissolution Bill in 1950, and advocated closer relations with Southeast Asia as a defence against the domino effect. She led several Australian delegations to international women's conferences, and spoke frequently on women's and children's issues.^[1] With several of her Liberal colleagues, Robertson lobbied for the removal of the marriage bar in the Commonwealth Public Service; this was not achieved until 1966^[11] Another pet topic was the conservation of historic buildings.^[10] Robertson did not re-contest her seat at the 1961 election,^[1] and her final term expired on 30 June 1962, a month before her 80th birthday.^[12]