[1] He then returned to Australia and registered with the Medical Board of New South Wales on 25 March 1872[1] and established a successful practice as a physician.

[1] In 1882, he was appointed the first president of the newly formed Board of Health, which brought him in touch with the poor of Sydney and the conditions in which they lived.

[4] He was vice-president of the Executive Council in the ministry of Sir Patrick Jennings from 26 February to 23 December 1886, and then Secretary for Mines until the government was defeated on 19 January 1887.

He consequently opted not to stand for a full term at the 1903 federal election, and not long afterwards resumed his seat in the Legislative Council of New South Wales on 26 November 1903.

[4] Mackellar had been chosen as president of a Royal Commission to investigate causes of the decline of the birth rate; he was largely responsible for the report that was issued.

He still, however, retained his interest and in 1917 published an open letter to the Minister of Public Health on The Mother, the Baby, and the State, and a pamphlet on Mental Deficiency, which shows his clear grasp of the subject was still apparent.

Mackellar's health and memory started to decline from 1923 and he died at Sydney, on 14 July 1926; he was buried in the Anglican section of Waverley Cemetery.