He held ministerial office in the Lyons government as Vice-President of the Executive Council (1932–1934), Minister in charge of Development and Scientific and Industrial Research (1932–1937), and Postmaster-General of Australia (1934–1938).

[2] McLachlan was raised on his father's sheep-farming property at Naracoorte and received his early education at home.

McLachlan went on to study law as an articled clerk, initially with Davison and Daniel in Mount Gambier from 1890 and then with Eustace B. Grundy's firm in Adelaide.

[2] McLachlan ran unsuccessfully for election for the seat of Victoria in 1896 and for Adelaide in 1912 in the South Australian House of Assembly.

He also ran unsuccessfully for the Legislative Council in 1905, the Australian House of Representatives seat of Adelaide in 1908[3] and 1910[4] and for the Senate in 1922.

[1] McLachlan became an outspoken advocate of military preparedness and supported sanctions against Italy in response to its invasion of Ethiopia, antagonising cabinet.

He also pressed Lyons over his unwillingness to put into place the provisions of the National Health and Pensions Insurance Act 1938.

[2] He unsuccessfully sought Liberal and Country League preselection for a further Senate term prior to the 1943 federal election, but recorded only eight out of 200 votes from preselectors.

[7] McLachlan was active in the Scottish-Australian community, serving as chief of the South Australian Caledonian Society from 1899 to 1902.